Wrath of the Lich King.
- The Class Balance - Ignoring the Deathknights, and for a few patches the Paladins, it was the most well balanced in terms of how classes stacked against each other WHILE still having each class feel different and unique. Every spec was viable and there was no class stacking like there was in The Burning Crusade, hybrid classes had competitive DPS but were still a bit below pure DPS classes, as it should be.
- The Raids - had two of the best raids in the game: Ulduar and Icecrown Citadel. If you've never gone through Ulduar, I feel bad for you, the experience was simply amazing; the bosses were all differently themed and each had their own challenge to them.
https://www.youtube.com/watch?v=xEylX2LJ8c4- Play Style - Had a reasonable balance between casual and hardcore. While Vanilla and The Burning Crusade were at times unforgiving with the amount of time you needed to invest to do raids and Cataclysm onward were far too casual, Wrath of the Lich King was a decent difficulty, yet still allowed hardcore players to be distinguished amount the crowd of people.
- The Lore - What's that? Zombies? Dragons? Old Golds? Having the story of Arthas come to a close and huge character development, reveals, and background. The story of Wrath of the Lich King (Trial of the Crusader was filler content, even Blizzard admitted it, so don't bring it up) flowed so smoothly and kept you excited and wondering what was going to happen next. There were large quest chains that revealed so much about the Lich King, the Dragons, Titans and the Old Gods that it was almost overwhelming, but in a good way. And let's not forget...
https://www.youtube.com/watch?v=Ch4rc5W4dKY- Achievements - now hear me out, I don't mean those baby-first-step achievements like getting a haircut, reaching level 10, or not falling out of your chair and breaking your spine, I mean the ones you had to go out of your way to achieve. Be honest, would you have bothered exploring every part of a zone if you didn't get points for it, would you have tried to complete 98% of the quests in the game if you didn't get the awesome title of "Loremaster", would you have murder all the critters of Azeroth were you not already completely mental? To me achievements allowed you to pour time into the World of Warcraft where you normally would not have, making it so there were things other than farming, or raiding, or pvp. They say variety is the spice of life, and if that variety has me trying to kill 15 turkeys in 3 minutes then so be it!
- Wintergrasp - that is all.