Zones and Quests
Entering the Outland was a disappointment, I guess I was expecting more than a red Desolace. Zangarmarsh with the immense amounts of running around didn't change that opinion any better. I despise most of the zones, par Nagrand, which especially for a Horde character was excellent, both quest and lore wise. All in all not a pleasant atmosphere. Shattrath, on the other hand, was excellent. I don't feel like it robbed people from the Azerothian cities due to the lack of AH, but yet it provided a place to start adventures from. It's like a big marketplace without anyone selling anything, but the customers are there.
Tubes are not into my liking. CoT was excellent. Heroics were an excellent addition and added longevity and alternate sources for gearing up. Badge introduction (I'm counting it into the 5-man stuff because that's what it originally was) was great, providing a way to get some bits of your gear that just wouldn't drop in Kara / instances. When they were made widely available and the gear attained ramped up to match, and in some cases beat t6, it wen't downhill. I was also fucking disgusted to grind all the badges to attain Fire Res gear to tank Leotheras ^_^
I only raided up to Illidan and quit after that so I can't comment about SWP because it wasn't released at the time.
Kara - Excellent. A nice, clean entry level raid with fights that introduced new raiders to the game mechanics.
Gruul - Loot issues (DST) aside, a nice introduction to the co-ordination required in 25-mans. Perhaps a bit brutal pre-nerf.
Maggy - Very, very demanding for those who couldn't beat Gruul pre-nerf or went to it straight out of Kara as their first 25'er. Personally I enjoyed the fight a lot.
SSC - Ah... the cockblock of my guild back in the day. Vashj. We spent over 2 months on her. That doesn't mean that it isn't one of the best fights in the game though. The other bosses felt a bit trivial.
TK - Home to the best bossfight in the game according to many, and I can but agree. The difficulty might've ramped up a bit too quickly from VR to KT, but the time spent learning that encounter and finally nailing it was the best I had in TBC.
MH - I enjoyed the trash system, but that's propably because I play(ed) an AoE heavy class. The bosses were a bit unimaginative (spelling?) par Archimonde, which again might've been too much of a stepping stone after the 4 bosses before him.
BT - It felt like the bosses had no connections to each other. I fight a big fucking naga throwing spears, and then I'm slinged to battle an elemental the size of 2 houses? Who are these guys? What? Illidan was a disappointment, having the fight focus more on adds than Illidan himself was a wrong choice of design if you ask me.
I'm a casual PvP'er at best, and as a warlock I had exactly one viable spec throughout most of the BC. Add resilience and the experience wasn't good. The little arenas I did seemed to consist of facing the same fotm teams over and over.
Shitty design. I wore my FSW chest until I quit, because the only alternative I would've accepted never dropped off Vashj. Others have commented on the issue more than enough.
Best and Worst
Best: Introducing raiding to those with less experience.
Worst: Raids, and in some cases instances felt torn apart. There was no longevity, no proper story.