Dangerous question, you'll probably get several more answers than the number of people actually answering

. I'll give you the non-technical answer, 'cause that's the one thing I'm definitely good at in this game

.
I'm much more comfortable with a ranger, probably 'cause I started with one, and Pantheo (he's my second) has about 700 hours on him now. I like running around with my Kitty, but it's hard to find good pet builds. I guess I like being able to attack at range, but an ele can do that too. Completely not a reason to choose a particular class, but I like the ranger armor best (well, some of it...), and that's a personal preference anyway. Interrupts are useful, 'specially if you can daze a caster or stop self-heals. Though from what I've seen mesmers are better at interrupting. And to be honest I can't explicate much more past I just like it. (maybe if I didn't suck so much with words)
Elementalists definitely do a lot of damage >_<. And they have a -lot- of energy, though it tends to get used up (which may be different if one is any better at playing an ele than me

). Blowing things up and setting things ablaze (especially ice) are definitely fun. NPC eles are generally pretty scary, so a good human ele would be more so. And... that's about it, I've not played mine very much.