As a Vanilla player, I share much of the same sentiments as you do. That said, I no longer play WoW on other than private servers, so here's what I have to say about it

Community Unless you are hunter or warlock, you will have problems doing a group quest. However, meeting just 1 other player and being forced to group up with him might make it possible, and with 3 it's almost a sure-fire deal. The importance of community is so relevant when you play a server that is based on Vanilla WoW, and that is something modern wow very dearly lacks. This becomes even more important at max level when there is just one server and no server transfers.

Class Homogenization As you said, classes have gone long way since their inception, and as you said, I understand also that this was done so every class would be viable/balanced for raids. However, here comes the mistake; raids have been typically only enjoyed by a small % of the playerbase. Balancing raids around them is a very major mistake, and I can only assume at this point the developers simply didn't know what they were doing.

Class homogenization and balance issues extend to PvP as well. Almost every class has mobility, CC just for the sake of giving them CC to make them more viable in PvP, wich totally breaks the immersion for many a class.

Old talents The new talents are absolutely awful. Im not saying we should be running with 80 talents now, in fact quite the contrary - in my opinion, the level cap should have never been increased to begin with. But the way Vanilla talents relate to the feeling of progression and love for your class as well as flavour, was awesome. They didn't have to be strictly balanced, as they weren't in Vanilla; as long as each talent provided interesting option for the player, they were fine. Furthermore, it's the potential that Vanilla talents had wich really interests me, wich materialized itself sometimes when warriors were running around with that silly revenge spec in WotLk, for instance. The old talents represented freedom, and the developers were so horrified about the balance issues that could be caused by this freedom of choise, that they decided to outright remove them, instead of allowing all the fun of customization that your character could have had with them.

So, the problems were basically two-fold, first increasing the amount of talents to make people buy expac, and then trying to pigeon hole everyone into using the same 51/41 max talent point for their spec.

Dungeons When it comes to dungeons, there was definitely a lot of choise. You had 5 man dungeons, 10 man dungeons, 20 man raids, 40 man raids.. that were different, instead of running the same treadmill 4 times in a row. That was important for immersion.

I fully agree that immersion is such a huge part in MMO's. It's pretty much gotten to the point where I concider myself not even in the target audience of WoW; it seems like they are aiming to make game for casuals and raiders only. As hardcore player, it's very hard for me to take modern wow seriously. It's just not a fun game for a hardcore RPG'er like me.