When I first started questing in Hellfire Peninsula, I felt like I was visiting a mall two days before Christmas. Everyone was after the same thing, pushing and shoving, and yelling at their friends across the zone/store. Alliance and Horde were being relatively peaceful, but there was no immersion, no beautiful landscape, no herbs to pick -- just a ton of people fighting across an extremely red wasteland.
So I took off for Zangarmarsh and Terokkar Forest. Here, I encountered a different problem: emptiness. There was no one to quest with or answer questions, and if you got attacked by multiple mobs, you would die alone (as opposed to in Hellfire Peninsula, where any untagged mobs are quickly taken off you.) It was pretty creepy, and I hightailed it back to HFP, which had quieted down a bit from the original gold rush.
Realms are a bit like these zones. Some, like (for example) Mal'Ganis, are so overcrowded that you have to wait in line for hours to get in. Sure, you've got a vibrant culture and playerbase, but the populations makes the servers laggy and queuetastic. On the other hand, you have dead servers that have lost most of their players to transfers or rerolling, where there are one or two raiding guilds and you can't find a group for anything. It's hard to say which is worse, but I think I would prefer the overcrowded realms to the dead ones. The free realm transfers might do a bit to alleviate this problem, but if everyone from one server transfers over, it could make it even worse.
What's your server's population like, and what do you wish it was?