Dual Talent Specialization
It's hard, if you started playing WoW after patch 3.1, to really understand how dual-spec altered the game. Before it, you were locked into your talent (and thus, in many cases, role) choice. Even if you collected gear for more than one role (as many did) in order for them to actually fill another role (like a healer going DPS or tanking, for example) then you were going to have to travel back to one of the capital cities, find a class trainer, and take an ever-increasing costly respec to wipe your talents and let you choose others. And it didn't save those talents - every time you would respec (say you were a DPS who was often asked to offtank) you would have to plug all those talents back in by memory or use an addon to store them.
Dual Talent Specialization changed a host of issues. People who had previously attempted to quest in healing or tanking specs could now switch to a DPS spec for soloing. Switching for instancing or to help your raid was now much simpler. A respec was now more often used for picking between talents, and not something you had to do to PvP, or run instances, or raid, or what have you. It altered the game tremendously, and it probably contributed greatly to the popularity of hybrids like paladins and druids. It made the "can tank, DPS or heal' aspects of those classes a good deal less theoretical.
I've said it since it was introduced - reforging (which was introduced as a feature that would allow players to make use of gear that dropped with otherwise undesirable stats, so that if a crit/expertise pair of pants dropped a tank could reforge some of the crit off and use them) has absolutely changed everything about how we gear our characters. In terms of personal optimization it is by far the biggest feature of Cataclysm and one of that expansion's great legacies in terms of how we play the game now.
It's had some negative impact as well - while I'm generally very positive towards reforging, I am cognizant that we no longer look at individual pieces of gear but our gear as a whole and how we can best arrange our stats. We use sites and addons to do the sometimes punishing math of our statistics-jenga, building our stacked towers of crit or haste or whatever your particular classes' stat priorities are. Reforging has become a huge part of how we optimize and select our gear.
I'm including all sorts of matchmaking tools here - be they PvP queuing, dungeon finder or looking for raid grouping. I could have just as easily called them "Queue for X" systems, be they battlegrounds or dungeons or raids or scenarios, what we're talking about (and what World of Warcraft didn't have at launch) is a way to enter content solo or in a small group and have other players added so you can complete that content. The first stirrings of it were at the battlemasters outside places like Alterac Valley or the portals in Arathi Highlands - interacting with them allowed you to place yourself in a queue and once that queue was filled, you would be swept away to a full 40 man AV or a 10 man WSG. It was the first stirrings of what we'd see flower in Wrath of the Lich King, as PvE content got an overhaul to its long out of date looking for group feature.
What had been a chat channel became the dungeon finder, allowing players to randomly queue for five man dungeons. You could bring a group of five and use the new system to queue for any dungeon you qualified for, or queue solo and be placed randomly in a group by your role, or any combination of people. (I often queued with three other people in Cataclysm for the Hour of Twilight heroics, as we would be trying to gear up one person by having two or three OP people queue with her or him.) This eventually led to the Looking for Raid feature at the end of Cataclysm, another contender for 'biggest feature' from that expansion. It's now possible to randomly queue in almost any group size available in WoW from a small scenario group to a 25 man raid. You can play WoW as a completely solitary person and still experience just about all the content, if not all of the content difficulties.
Bind to Account
When the game launched, there were bind on equip items that you could trade, sell, or use as you saw fit, and bind on pick up items that were soulbound to your character as soon as she or he picked them up. And that was it. That was how it went - once you'd put a BoE item on, it was bound to your character who was wearing it and only that character, forever. And if you picked up a BoP item you didn't want or need, well, tough - it was bound to you now. It often required a GM ticket to get those kinds of mistakes sorted out. (This was later improved by making it possible to trade an item to anyone who was eligible to loot it for two hours after it dropped.)
In late 2008 players discovered Bind to Account items called Heirlooms that would not only be tradeable between characters on the same account (so your paladin could wear them, then take them off and send them to your warrior) but which in the case of the armor pieces also gave an experience buff. These pieces also leveled up as you leveled to a point (usually close to or at max level) so that you wouldn't want to keep them or use them on your first character. Heirlooms themselves would have made this list, as they completely changed alt leveling, but BoA items transcended that - soon the new Archaeology profession included non-heirloom BoA items that players could send between alts as well. These items do not level as you do.
The potential of BoA items and heirlooms still has places to go - we've seen that they're experimenting with heirlooms that will drop from bosses and level up throughout the next expansion on the PTR. It's definitely a kind of gear lock that we didn't expect when it first happened, and it has already changed the game substantially over the past couple of years.
I'm not going to waste a lot of words on this one. Instead, let me show you a picture.
We used to have to look like that. Now we can choose to.
So there we go, some features that have utterly changed the game. There are more: let's hear them. I know you're itching to point out stuff like Arenas, jewelcrafting/gemming, battle pet system... so do it. What're your biggest game changers? (Also, I dare you to find the Bill Hicks references in this post.)