Rohan from Blessing of Kings has a brilliant short post on leveling speed, in which he observes that everyone thinks he has exactly the perfect balance between being a no-lifer and not being dedicated enough to be leet. But while that is as funny as it is true, I nevertheless think that there is also a good amount of personal choice involved.
My priest is still level 73, in a guild where the highest level guild member is 78. And I only got to 73 by taking Thursday and Friday off from work, and playing WoW all day. I draw a line at playing all night, I need my beauty sleep. :) But it is certainly true that I gain less xp per hour played than some other players.
I could cite many reasons for that, like me being an explorer, or me spending time to do things like leveling up tradeskills, or just traveling around, or doing other activities which might not have the perfect xp per hour ratio. But the fundamental reason behind all this is that I see WoW more like a game, or even toy, than like work or a sport. It is not that I wouldn't be capable to gain xp at a faster rate by doing more research, or concentrating on certain activities; it is that I wouldn't have that much fun doing so.
Nevertheless I totally understand the motivation of those leveling much faster than me, the achievement of server firsts, or wanting to get into raiding as fast as possible. I also understand the other extreme, for example my wife, who since I installed WotLK for her is soloing elite quests in Nagrand, saying that she doesn't like Northrend as long as it is so crowded. The thing is that there is no "one true way" to play World of Warcraft. In fact it is a big strength of the game that it can be played in so many different ways. Now if everybody understood that, and stopped calling each other no-life or slacker, WoW would be an even better place.