One of the first things that you're going to have to wrap your head around is that your starting class determines nothing except your starting equipment. Every character is every class, not just eventually but from the start of play. Or to put it more simply, leveling classes in FFXIV is much like leveling weapon skills pre-Cataclysm.
The system works thusly: You pick a class, which determines your starting armor and your starting in-hand equipment. Whatever you have equipped in your hands determines what class you are at any given moment. So as long as you have a sword in your hand, you're a gladiator. Swap that for a two-handed axe, and you become a marauder. Use a wand for a while and you're a thaumaturge, and so forth. As you level with a given weapon, you unlock skills at a fairly steady interval, and the vast majority of skills can be used by any class.
It gets stranger -- you also have a limited number of abilities you can use at any given time. Depending on your rank in a given class, you have a certain number of action points, so a rank 3 pugilist isn't going to be able to use as many abilities as a rank 12 marauder. Even though most of the marauder abilities can be used by the pugilist and vice versa, the number of AP available will only allow you to use a few abilities.
For many players, the easiest way to conceive of it is that you have multiple talent trees. As you level each class, you unlock more talents for use, but you only get so many points per level. That forces you to juggle some degree of efficiency as you level -- do you focus on a single class and have more strength with that class with a narrower focus, or do you level several classes to be a jack-of-all-trades?
Also worth noting is that you do have a physical level, which affects your base attributes such as strength and intelligence. This level is increased with experience points from killing enemies, completing quests, and performing pretty much any activity that will get you skill points. It's straightforward and unchanging, like your level in WoW, with the small caveat that your stat points have to be manually assigned when you level up.
Being a high-level crafter in WoW has some notable effects. Gathering skills each give you a small passive buff, and crafting skills allow you a variety of bonuses that are specific to the crafter. Final Fantasy XIV takes that a step further, with crafting and gathering both taking up their own classes altogether. You don't just have, say, a warrior who also knows alchemy -- there's a separate class for alchemist, complete with specialized tools and equipment.
Crafting and gathering are both going to be almost completely alien if you're accustomed to the WoW model. Both involve small minigames that determine success or failure. There are also no built-in recipes for players to select from -- you select your materials first, and the game will inform you what (if anything) can be made from those resources. There's also the issue of quality, with crafting quality being the product of careful work and always a bit of a gamble.
We'll have our own guide to gathering and crafting available tomorrow, but suffice to say that these two activities are not simple sidelines for adventuring in FFXIV. I'd encourage you to at least try these two systems out when you get in the game, however, as they also provide some of the most distinct parts of the gameplay. And yes, if you always wanted to level a crafter in WoW without doing anything but crafting, you can certainly do that here.
If you've been playing a rogue for the past six years, you'll be in an excellent place for combat in FFXIV. It's not identical, but very similar.
All classes share the same three resources: Stamina, TP, and MP. All actions consume Stamina, which is constantly replenishing and works very similarly to energy. Your goal in combat, much like the goal of a rogue, is to never max out your stamina mid-fight -- which is still easier said than done with the regeneration rate. MP is consumed by magic, and TP is consumed by a variety of special moves, usually weaponskills. (We'll get to those in a moment.)
The biggest difference is that unlike WoW, FFXIV has no auto-attack. Each discipline of war (gladiator, pugilist, et al.) has two attacks to choose between, while disciples of magic have a basic magical attack to use. No attacks will auto-fire, so you have to keep either selecting the icon or pressing the key to activate the ability. If you traditionally click on the action bar, FFXIV is wonderful motivation to make better use of keybinds.
TP works along the same lines as rage, building up from zero as you hit your target with physical attacks. When you have enough, you can consume it for weapon skills, which are generally high-damage attacks with secondary effects. (It's somewhat akin to Stormstrike or Mortal Strike.) There are also a few other abilities that use TP for other effects, such as quick self-healing or the like, along the lines of Frenzied Regeneration or Death Wish.
And speaking of death, now's a fine time to discuss shuffling off this mortal coil. Similarly to WoW's tossing you back to the graveyard, you'll wind up needing to return to the last aetheryte node you touched, at which point you'll be automatically raised. Items do not lose durability on death. Where WoW's item durability is the main penalty for dying, FFXIV's items simply lose durability gradually and require repair over time. Death in FFXIV will leave you briefly weakened, which lowers your stamina generation, HP, and MP by a notable percentage. It wears off after a couple of minutes, but it does put the skids on any repeated zerg attempts.
At least as of this point in the game, the primary method of transportation is teleporting amidst the aetheryte crystals. The basic concept should be instantly familiar to anyone who has spent some time linking flight paths across a continent -- in order to teleport to a crystal, you have to first visit that crystal and attune to it. Once you've done this, it will be added to your list of destinations.
However, aetheryte's ease of use makes flight masters look like the Greyhound Bus of tranportation. (To be fair, they kind of already are.) You can not only teleport between Aetheryte nodes freely, but teleport to any node you've visited, regardless of how far away it is. There's no need to link paths a la flight masters. Any node you've visited will be available for teleportation at any time.
Well... for the most part, anyway. There are two kinds of stable node in the game: the large crystals with several smaller rotating crystals, and the tall but smaller single crystals. The latter does require you to be at a node for transit purposes, but it doesn't cost anything to teleport between the smaller nodes and the large hubs within the area. It does cost anima to teleport across the continent, but anima regenerates on its own slowly.
Questing in FFXIV, much like in WoW, is what players are expected to be doing with the vast majority of their playtime. That much is constant. However, unlike in WoW, there are a wide variety of different types of quests in FFXIV, ranging from levequests to local quests to the ongoing main storyline. None of them quite follows WoW's format of having you pilot to a quest-giver with an exclamation mark and so forth.
Levequests make up the bulk of the questing that individual characters will do, and they're similar in some ways to daily quests. Much like some dailies, they're acquired from a central location -- the difference here is that the central location is at the Adventurer's Guild in your city of choice, and each one points you to a specific quest hub. From there, you go to the aetheryte where the levequest takes place and then begin the quest.
Like the quests in the starting zone for death knights, levequests make use of phasing-style technology -- the targets of the quest can only be seen by you and not by any other players. Similarly, your minimap will guide you in the right direction. The best equivalents are quests dispensed in Stormwind or Orgrimmar -- you take the quest, fly out to the nearest flight point, and then head over to the actual quest area. The only major difference is that you have to specifically start the quest at the aetheryte before the enemies will spawn, and you will be given a time limit (which is almost always reasonable).
Each levequest has a small stained-glass card that comes along with it and is displayed when you take the quest. That's where things get interesting -- because while you can only do a maximum of eight levequests a day (much like the limit on daily quests), you can get increased rewards with new quests the next day. When you take a new set of quests, you are given the option to trade in the ones you already have used. The more quests you trade in, and the more different types you trade in, the greater the rewards of the new levequest.
Here's an example. Say on day one, you take four combat levequests, two mining levequests, and two botany levequests. The next day, you select a new combat levequest and are asked what you want to trade in. By trading the mining and botany levequests in for the combat quest, that quest will have increased rewards. The specifics are still being worked out by the community, but it's enough that you know you'll always want to exchange one levequest for a different kind of levequest.
The two other major types of quests are local quests (which give you the opportunity to craft items for a reward) and the main storyline quests. The former is an excellent chance to both learn new recipes and get some skill improvements for your craft of choice, especially since the crafting materials are provided as part of the quest. Main story quests, on the other hand, are less focused on rewards and more on advancing the overall story arc as you journey through Eorzea. The first few quests send you around the city you started within to get you acquainted with the area, so they're certainly worth doing.
The New World
There's a lot of stuff in FFXIV that plays very differently from WoW, and at times it can be a bit daunting. Unlike the vast number of games drawing clear inspiration from Blizzard's goliath, the environment of Eorzea can feel very strange and alien. But it also offers a rewarding experience for players who stick it out despite the unfamiliarity.
If there's one thing that the game does well, it's reward players for taking the time to stop and smell the roses. Don't worry if you're not totally sure how to play from the word go. Get out into the world and start playing, go with trial and error, and remember that nearly everything in FFXIV can be reversed if needed. It's a wide-open world, one best taken as a new experience.
Good luck and happy adventuring!