BM Hunters are unfortunately focused around pets, which die all the time in Vanilla. Their damage also does not scale very well with gear (if at all?). Honestly, you get most of the functions of being a BM hunter, minus the 31-point rotational talent, without having any points in the BM tree. BM is strong at leveling, though. Survival is a PvP spec, and Marksmanship is the best PvE spec the whole way through Vanilla, by a wide margin.
You also get the whole fun of “bonding” with your pet in Vanilla by feeding it, keeping it happy, leveling it up, etc.
Warlock is a curious case. Affliction DoTs in Vanilla actually do a ton of damage, but you can’t use them outside of leveling and 5-mans because enemies only have 16 debuff slots for the entire raid to use. That means, most of the time, your raid will do more damage by using curses that increase various types of magic damage taken.
Destruction warlocks do a lot of damage, but they are limited until AQ 40 because everything has high amounts of fire resistance. Before that point, as a warlock, you are mostly doing this cool min/maxing by summoning/sacrificing your pets to do buff management and casting shadow bolt. The non-fun part is you are mostly just casting shadow bolt until AQ 40.
Enhancement Shaman is one of the classes that I would say is unfinished in Vanilla. They have empty globals in their offensive rotation, because Stormstrike is on a 30-second cooldown for some reason. Their damage output is poor in PvE, but they are absolute monsters in PvP. Like all shamans, they get to use totems, which in my opinion, are the most fun class element in all of Vanilla.
Shadow Priests generally go half-shadow and part holy/disc. In raids, you provide the Shadow Vulnerability debuff, which is essential. But you also do poor damage, so you end up being a healer/DPS hybrid most of the time. You are very strong in PvP.
Moonkin is very poorly balanced, and their numbers make it so that you just spam one ability most of the time. In order to do respectable damage, they have an extremely high gear requirement, which is often not worth it. Again, good in PvP.