Optimization
Icecrown is one of the highest populated servers across wotlk. It's a stable choice if you are looking for a medium to high XP rate experience. One of the great things around Warmane - especially with all the Turtlewow drama - is that Warmane seems to just always stay up. No drama about Blizzard. No new surprises. Just silence from the ownership team and focus on the game itself.
Turtle-WoW (turtlecraft) is by far the biggest WoW Private Server around. They are a Plus vanilla based server with a great set of ingame features. The staff team is dedicated and are actively creating new content, hosting events and communicating to the playerbase. The server is loved by the majority and is definitely a must-try for new players. The only downside with Turtle WoW right now is the amount of attention it has gotten by Blizzard. In other words, it has almost grown too big. This reminds me of Nostalrius back in the days. So there's a bit on uncertainty right now around the future of turtle wow.