I understand from a game mechanics perspective the bosses need to be present so raids can take them down. But there's a story here, and I'd like to know why the boss is just sitting there in that room waiting for 10 or 25 heroes to come beat them to a pulp and take their stuff.
This to me is key to making the raid feel immersive and epic. Ulduar had some bosses that were mysteries but most of them were taken away from the quest zones and kidnapped/corrupted. It made freeing the keepers feel meaningful.
I also enjoy places more when they have decorations instead of catwalks filled with trash. You could literally clear Kara of trash and still feel like that would be a great guild hall. Kara could definitely have done with a lot less trash but it wouldn't lessen the epic nature of that place. The BT courtesan area is another great section where it feels lived in, used.
THe other thing that is key is trash that actually looks like it is fulfilling a purpose rather than just slapped in there as filler just because we have to slow players down or it fills the space. I can't stand 90% of the trash created because it just stands there all doe eyed waiting for adventures to stumble in.
BoT is a start back in the right direction with trash that is patrolling and has some interesting abilities but I'd like to see it go a lot farther. My favorite trash is Shattered Halls - you felt like stuff was coming in response to intruders, and the ZA trash gauntlets for the same reason. The courtesans in BT and the trash in Shadow Lab are good examples of trash that feels like it's doing stuff before you come in and interrupt them. Everything else has just been decoration filler and not worth my time as a player to spend on. I love Ulduar but most of its trash has the same issue.