I am at lvl 20 fractals right now, the professions I like to see are:
1: Warrior, Good cc and heals, and the only class that is able to go glass and not die every 2 seconds.
2: Elementalist, Able to keep up a lot of party buffs, also decent heals.
3: Guardian, Can do the same as an ele, the lack of a good ranged weapon makes ele the better choice for me.
4: Engineer, Great utility (Elixir R!), also tanky and good damage.
5: Necromancer, Just plain tanky, also the best class to dps the seal at the colossus fractal.
6: Mesmer, Useful in a low level pug, but not really needed later on, I expect people to be able to run a wisp when they reach 10+.
7: Ranger, Very situational, has to attack from behind to do good damage, RaO is a total boss at the swamp fractal though.
8: Thief, This just isn't a good PvE profession, most thieves also run glass cannon builds, and are as useful as someone running full MF gear.
I usually run with 2 other people, one runs a fairly tanky banner warrior with a hammer, the other runs a support elementalist focusing on regeneration and protection, and I am using the vigorous shouts build with full clerics and axe/shield.