FWIW, there is a long history of illegal private servers that offer Vanilla and specific xpac servers for WoW. They are especially popular with people that are into player versus player because people tend to have certain xpacs that they believe were more fun for a variety of specific reasons, mostly related to balance between different character classes.
Every expansion has a "best class", that's for sure. Even if you try to balance everything, that's impossible to achieve; every class has dozens of spells, talents, buffs... and then, some of those spells, talents and buffs can interact with other classes (especially when in party or raid) improving or nerfing them.
In that expansion I'm sure you're talking about, paladins are somewhat better, and shamans are worse. That's normal, and it's not a problem when you play in live servers, since they are making modifications to spells and everything constantly. But when you're stuck in some version, then, you're stuck with how classes work in that version forever. That makes the population lean towards one class.
