I wasn't so sure if that was the case. I really do love priest healing, and have devoted alot of time and effort to learn as much about it as possible to be as good at it as possible. It is without a doubt the class I have raided the most with. But is it the class I have played most or do play most, off-raid? Definitely not. One big problem about healing is that as soon as you get good gear it gets too easy. Healing heroics is probably the least-action filled thing you can do in WoW today.
Some years ago this one-class thing was more the rule than the exception. Blizzard didn't originally intend for people to respec on a regular basis, and they probably never ever dreamed of implementing the dual-spec system. At first the respec costs accumulated indefinitely, making it extremely expensive to respec your character after a while. If you wanted the same class with another spec it was often better to just roll a new character (which I often did, and the habit stuck with me until dual-specing). They quickly noticed however, that being able to respec your character was one of the most loved features of WoW, one that was much in use even though it was so expensive. So they put a roof on the expenses (50g). But you could still only have one spec, and some (like Love, who according to armory has respeced 175 times until now) who played hybrids or just liked respecing, respeced several times a week and still had to put alot of money into it. Others, like me, simply rerolled a new character with the desired spec. That's for example the reason I've played four shamans to 70+ (one resto, two elemental and one enhancement). And then Blizzardfinally caved in and gave us dual-specs. But I would easily pay 20k g for the possibility to have triple-specs.
So back to the question; If I could play only one class evermore, which would it be?
I really love priest, but like I said it quickly gets too easy (this might, hopefully, change with Cataclysm however). And although I have played alot of shamans I wouldn't like being able to play only them. Shamans are simply really fun to level, but not so fun at end game. That is probably because they, just as paladins, work really well for several roles while leveling, but in endgame they're rather boring (at least I think so). One class that never has bored me, not while leveling, not while raiding and not while doing heroics is (you can probably guess it) prot warrior. Like I've mentioned before, tanking well is always an effort since it's really up to you and the rest of the group how difficult it will be. If the group is really well geared, keeping aggro will be a handful. If keeping aggro from the group isn't too hard, you can instead try to make big and cool pulls to keep you occupied. I'd definitely miss priest healing, and I do think healing is way more fun in raids than tanking. Tanking is probably the easiest thing you can do in a raid, mostly (there are always exceptions). Healing might not be the most difficult thing (I think so, but I realize some people think it's sooo hard to keep track of a rotation and the environment at the same time, boohoo), but it's definitely the most changing thing you can do. What a dps/tank has to do when they have to interrupt or cc is what a healer has to do all the time.
But this isn't a debate about which is the more difficult. In the end it's about which is more fun. To me difficult, or rather challenging, equals fun. And I believe, as it is today, I would have more fun with a tank warrior than a healer. So I would either choose a prot warrior to play alot of heroics or choose a priest healer to do a few raids (or perhaps alot).
Which class would you choose if you could only play one? And why?