As was probably obvious in our discussion of world setting above, Warhammer is a much more 'serious' place than Azeroth. You won't find pop culture references thick on the ground, no Paris Hilton stand-ins selling merchandise in a tavern. This isn't to say that Warhammer is without humor. Quite the opposite, actually - the game is full of black humor and fart jokes. The tone is just very different than what you're used to from WoW.
The graphics are an equally big departure. Bright colors and rounded edges are the exception in Warhammer, not the rule. The most 'pleasant' places you're likely to see in Warhammer are in the High Elf areas, and even there the ravages of war have left their indelible mark. The world as a whole is a pockmarked land of old battlefields and burned villages. Colors are muted and toned to match.
Instead of the typical yellow exclamation points, players new to Warhammer should be on the lookout for green circles. They're hanging over the heads of NPCs with stuff to do, and much of the questing experience in WAR will feel very familiar to a WoW veteran. What is different is that your questing experience creates an actual story for you, which you can follow via the Tome of Knowledge. Looking back through the Tome at past quests you've accomplished gives a real sense of an ongoing tale, one which is well worth exploring.
There are also a few different types of quests in Warhammer Online, most of which are fully explored in the famous 'bears bears bears' Mythic podcast. Kill Collectors have been implemented in the game only for certain creatures, and the collectors are located in specific hubs. Look for their distinctive markers on the map to get credit for your kills. The most talked-about new quest type is the Public Quest, an epic event that bands numerous players together to accomplish a unique story-based objective. Be sure to read up on PQs in one of the couple of features we've done on the subject.
Though WAR crafting will feel somewhat familiar to WoW players, the act of crafting occupies a very different role than it does in Azeroth. Crafting in Altdorf and Inevitable City isn't the full-fledge hobby you have in Stormwind or Undercity. There are only two actual creation crafts, potion-making and talisman-crafting, and a handful of collection/gathering crafts. Full details on the WAR crafting process can be found in our in-depth interview on the subject. Suffice it to say that if crafting is your favorite thing about World of Warcraft, you might be a bit disappointed by its backseat role in Warhammer Online.
Players who are only used to the 'chat channel and a tabard' will be very pleased by the numerous options and advancements available in Warhammer Online. As players advance, so to do guilds - unlocking new abilities, privileges, and rights. Unlike the sometimes-fleeting state of guilds in other online games, the Mythic designers are very firm in their wish for guilds to be fixtures and encampments. Permanent structures to which old players can cling and new players can enjoy. We have an exhaustive interview with the developers on this concept as well.
Warhammer's user interface is as customizable as Warcraft's, and within months you're sure to see user-generated mods from the community. Alongside those external mods, though, Warhammer features a user-changeable interface. Elements can be rescaled, resized, and moved with almost limitless freedom. However you want the screen arranged, you're going to be able to create the experience you need. Each class, too, has a unique UI element on the hotbar at the bottom of the screen. This UI element ties directly into class's mechanics, and informs you how you should be playing the game. It may not surprise you at this point to learn that we have a quick post about exactly this subject.