This is mostly just my opinion, so take it or leave it.
Specs being nonviable I guess I could give you, but classes weren't at least. I can't think of any class that saw zero use in the game. Weapons skills were interesting for me, even on caster classes. It gave a sense of "I'm some new adventurer in this strange world. I've never even held a sword, so maybe I should practice so I'm really good at it." At least for me, a lot of the inconvenience made the game fun and interesting. Running back after a wipe meant time to discuss what went wrong with a pull/boss and what to do differently next time. I learned a lot, like the patrols after you cleared the foundry in Deadmines sneaking up on you. Spamming trade for a group and running there usually meant the group would stick together because the time commitment to do a dungeon was pretty high, and getting someone to replace a player was a task in itself. I don't really remember anyone leaving my groups back then with the exception of BRD since it was fucking huge. I don't think I ever killed the final boss. Just ran it like mad to the Grim Guzzler and to get my MC attunement done.
Attunements totally added another feeling to the world. "I have to do all this stuff to even go into this raid? It must be pretty awesome/dangerous in there!" I loved attunements even in TBC. Getting my Kara key felt like such an accomplishment. It gave the raid itself for depth because of the tasks I had to complete to get there.
Overall, the experience was much more enjoyable compared to current retail. I still enjoyed raiding with my mythic group, but outside of that there wasn't really anything for me. I didn't really know anyone outside of my guild, and never really had a reason to. I knew quite a few people on my old vanilla realms from dungeon running with them, or pvp, or just some nice guy offering a carry through SM-Armory.