Believe it or not, there are few things in the game that have annoyed me as much as the way gaining wep skill used to work. For any weapon you could use you had to not only go buy the skill in some cases, you had to also manually learn it by swinging at a mob with said weapon until you reached the proper skill level. Going from 1 to max skill could take hours.Yeah, it was pretty miserable. Casters wouldn't bother. With melee classes, though, you have to skill them up, and it takes forever. The first few points go quickly, but it really slows down when you get close to the skill cap. RIP, weapon skills. You were a terrible bore.
It got me thinking what I am happy has changed. Here are five picks of my own.
The Dungeon Finder. I played a little bit in vanilla and had no idea what was going on. I found 5-man instances one of the most fun parts of the game, but I found it really tough to find an instance. I came back to the game in Lich King, and I still knew what was up, but assembling a group for anything took a while. The Dungeon Finder is a huge improvement, especially for dps classes. Instead of plaintively asking for a group for half an hour or more, you hit the front of the queue in about 15-20 minutes, all without having to beg and plead.
Newbie friendliness: tooltips and the new spec system. Jinxed Thoughts mentions this one, too. I like that the tooltips say things like "a large heal" rather than giving a very precise and very bogus number like "heals for 132 points". The spell always healed for more, because the tooltips didn't factor in all the buffs that apply, so the number on the tooltip was pretty useless to anyone who is not a human calculator. Likewise, I like the new talent system, where you get spec-specific improvements right when you pick a tree. It means that level 10 is way more fun than it used to be.
Easy mode of raids. I like that casual players like myself can at least see a softball version of everything.
The new token system. The new token system means that it's fairly quick to level up an alt to being able to work on the previous tier of content.
Class balance. I'm impressed and glad that every class has a competitive dps spec, and every healer and tank class is about equal to the others on the whole. Getting a character to level 85 takes a while, and I'm glad that nowadays you can be sure you won't regret your choice of class.