Jun 4th 2009 8:19AM I think there's a balance between being able to get groups easily and your guild being dependent on you because there's a shortage of tanks or healers. The latter makes it hard to play casually in a fun way if you are feeling you are letting folk down when you choose not to sign up for a raid.
Another factor is that learning raid encounters to a reasonable level is also harder work for some classes than others. Sure, there's effort and skill if you want to max your DPS but there's also less potential to cause a wipe directly.
In practice, I just make a point of trying to only fill raid slots that would othewise be empty so I feel like I am contributing something useful even if I'm not as well-geared or prepared as the more serious players!