Burnout is pretty common amongst MMO players. It seems to be even more apparent since WotLK since most all content has been conquered by many guild (and even some pugs). If you find yourself logging in, putting around Dalaran for a few minutes and logging off, dreading seeing the green chat when you log in, or seem to get irritated over any small thing that does not go right, you may be suffering from burnout. There are many ways to combat this.
1. Take a Break: This is a hobby. It is something that we do for fun and usually a sense of community. It should be something you look forward to. Sometimes, taking a break can breath a bit of new life into the game when you get back. Go hang out with your friends, take your lady to a movie, go to the bar. Whatever floats your boat. If you find that you hate baseball, do you keep watching it? No. Just move along and come back when you are ready.
2. Moderation is Huge: Playing the game too much is one of the leading causes of burnout. If you find that you are playing pretty much every day for extended periods of time, STOP IT (this applies to burnout only, if you are having fun keep at it if you want). Logging in to raid 5 days a week for several hours, plus farming time does not leave a lot of time for other things. Take it casual for a bit. Only play on specific days, or have some lazy days that you just do something fun like chasing after pets or achievements if you are into that sort of thing. I find that due to my playing schedule, I really look forward to the times I get to play for an extended time. I play Saturday, Sunday, and Mondays, with the rest of the days going to my fiance. I usually play about 10 or so minutes on off days, but it usually consists of JC daily and re-posting stuff on the Auction House. Playing less makes you actually enjoy the time you do get to play.
3. Roll an Alt: Rolling an alt can be very relaxing. You are low level, doing it on your own with little or no complications. I would advise not bringing this alt into your main guild. Either keep him unguilded, or put him in a friendly leveling type guild. In either case, it is low stress and lets you focus on the part of the game you enjoy....actually playing. WoW is not a job, and you should not feel like it is. Do you ever go to a movie and stress out about it? It is a form of enterainment and you should be entertained. Having an Alt outside of your guild gives you an opportunity to play minus the hassle.
4. Respec: When Dual Spec comes out, this will be easier than ever. It seems that the players who most often burnout are raiding tank and healers. If you find that you are sick of it, switch to your pew-pew lazer beams spec. It really can be amazing how much fun and how relaxing DPS can be. Watching those big numbers fly can make you enjoy the game again. I used to respec quite often in BC just to do something different.
5. Reroll: This one is more or less a different take on rolling an alt. In this case, you completly switch to the dark side. If you are Alliance, try out a Horde toon. If you are Horde, do the opposite. Doing this on another server helps as well. You get to see a side of the game you may not have seen and basically get to know an entire new community. This may restore your zest for the game and make you realize that you should have been Horde/Alliance all along.
6. Try Something Different: If you are tired of doing the same thing over and over, STOP DOING IT. Raid, Raid, Raid, Raid, Farm, Farm, Raid, Raid is not for everyone. Every once in a while try something that doesnt taste like a stale piece of bread. Hit Wintergrasp, do a battleground, chase some achievements, play the auctionhouse, roleplay. If you find that you do not like punching yourself in the face IRL, you just stop. Same thing with WoW. It is voluntary. The time is yours and YOU choose what to do with it.
7. Dont be afraid to say NO: There is nothing wrong with saying no. A lot of people feel obligated to help when somebody asks. If you do not want to do it, just don't do it. Saying no can be hard, but in the end it will make you feel better that you did not go on that 10 man Naxx which has nothing valuable for you to wipe continuously with the guild's green geared scrubby alts. This is a game. You do not have responsibillity or obligation. You should not feel like you do. If you want to help, then do it. If you do not want to help, just say NO. If you say yes to every request, you will find yourself not doing anything you like and everything that person X,Y, and Z like.
This game should not feel like a job, a boring grind, or something that stresses you out. It should be something you enjoy and look forward to. If you find youself in the first category, perhaps it is time to start following the above steps to fight burning out.