Outdoor world was very dangerous, even low level mobs could overwhelm a max level.
Instances and raid were respectably tough, and there was a lot of class diversity - even though this caused massive balance issues.
PvE and PvP sets were one of the most beautiful in WoW, and overall the zones in Outland just looked amazing.
Whatever you wanted to do in TBC WoW, even just level alts, you met a lot of challenges and difficult experiences - you really had to work for your success, and it felt very rewarding.
The outdoor world pvp objectives were quite fun, and the quests and various factions and pre-raid dailies were quite difficult and required effort.
In my opinion, most of the interesting lore and stories happened during TBC - WotLK was fine, but nowhere near TBC in terms of immersion.
Raids had a good mix of various content, i did not feel the same boredom as i did during WotLK by constantly fighting undead or during Cata where i was always fighting some Old God stuff.