The most damaging and community-killing aspects of dungeon finder are the cross realm part and the port to the instance, IMO. So having it on a single realm with no port probably wouldn’t be the end of the world.
But I think people grossly overestimate how useful it would be for finding groups. People think they’ll always be able to just press a button and find a group like they do in retail, but in single realm, the pool of players to draw upon would be much, much smaller.
The LFG channel already does a pretty good job of matching up players who want to run a particular dungeon, IMO, and if a group can’t be found on LFG channel, it’s probably because there simply aren’t enough people that want to run that particular dungeon online at that particular moment. And single-realm dungeon finder is not going to help with that problem. It’s not going to conjure up players out of thin air.
Dual spec, IMO, changes the gameplay significantly. It’s part of the whole homogenization of the classes trend over the course of WoW’s evolution, which a lot of people think made the game more bland. Suddenly every hybrid is a jack of all trades that can fill every role with a quick spec swap.
The original developers obviously wanted players to feel relatively unique in their spec, for their character to have an identity tied to their spec, and for different specs to add flavor to the game. Not being able to respec (or respec easily) was a core aspect of RPG design at the time this game was made. They didn’t WANT players respeccing unless absolutely necessary, IMO. The inconvenience/cost of it is intended to discourage it.
Plus, dual spec makes raids significantly easier because players can swap specs from fight to fight, to add/subtract specced tanks and healers as needed. TBC content was not tuned with it in mind. And with the other modern metas, raids are already going to be much easier than they were back then.
TBC is the best version of WoW ever released, IMO. I think it will be just fine without those two modern features.