I tried mage's and they just seemed to be too easy... in power. I like the difficult classes but dont want to be the terrible class no one wants to have in there group. I'm only level 43, but you know. Having doubts.
warlocks are a great choice, but they have pets which also makes it pretty easy. but warlocks own in pvp and still do GREAT for pve. may be lot of fun for you, but maybe try a druid, they can a little harder.
When I played WoW I thought the warlock was really fun and somewhat easy, especially having the minions around. I no longer play, but from what I remember, if you want a challenge, don't go with the warlock.