Played WoW for years myself so can offer my opinion I think.
I played on the US servers because a friend of mine lives in Canada, I originally started playing in the summer of 2005 on US-Skullcrusher and was learning the game.
Eventually a new server was announced, US-Mug'thol and my friend migrated there (It was free as Skullcrusher was a heavy populated server). I had my doubts but I was on the servers because of him so I went, I was still learning the game and only had very few friends. My friend went because his guild did and he was already raiding end game.
So this was the start of Mug'thol for everyone and I remained on the server for a few years as did a heap of others, the server quickly became very populated but nothing as bad as the servers today (for example EU-Magtheridon).
Long story short I ended up quitting due to terrible time zone differences preventing me from raiding, I was a hardcore PvP player at 60 and I was only known for that, I couldn't raid at all but I know a heap of people and had fun.
Then I created a new EU account when my brother decided to play WoW so we rolled on EU-Balnazzar on Alliance since I had previously played horde, knowing no one and not having a heavy geared char was a bit odd but the main problem I had is having no friends but most importantly was when I began raiding later on I knew nothing about the other guilds past or the servers history of events and it didn't feel right. Eventually I quit and sold it.
Rerolled again when Refer a friend was first introduced shortly before WotlK so I decided I would use this and try raiding for a while on.
Ended up being the same old story, I missed my original server where I learnt to play, made lots of friends but I think it may have to do with the fact that I enjoyed the level 60 days far more than that of TBC and WotlK.
I ended up quitting for good about 2/3 months ago and won't be going back anymore.