My $0.02 from the playing around I've done on my elementalist.
Necro: I have found that when soloing, the necro blood line (life siphon, vampiric gaze, vampiric touch, etc.) are VERY handy for keeping my elementalist alive, but only if you are going to take it high (mine is 12). I haven't played with the other lines, but I suspect curses could be fun.
Monk: When grouping, monk secondary is VERY handy. I nuke until we start getting beat down, then start tossing out heals. True, even with 12 healing the heals aren't as good as what a monk is capable of (if specced for max healing), but an elementalist can throw a lot of them with energy storage. Don't know if protection could be useful or not, but smiting seems almost redundant.
Mezmer: Has plenty of nice tricks that can both help you survive and manage energy as well as make the other guy's life suck, not to mention just being fun to play if it's your style.
Ranger: A pet COULD take shots for you, if/when you can get him between you and the mob, but I wouldn't rely on it for keeping me alive, and forget about it in PvP. Real players will ignore the pet and make a beeline for the squishy, paper-wearing healer/caster every time.
Warrior: Warrior will allow you to actually deal some damage with a melee weapon, but this requires you to be standing toe-to-toe with a mob, something that you should avoid like the plague and I assure you, any mob anywhere near your level will do more damage to you're squishy butt then you'll do to it with a sword. Probably ANY offensive spell you have would do more damage.
I haven't tried EVERYTHING possible, but these are the general impression I've gotten from what I have tried. I find myself using monk most of the time (when grouping especially) and necro when I'm soloing/farming. Of course, with the right skill set, you might find something even better, I would awlays encourage everyone to try it all. Once you ascend, changing professions is easy, and respecs are basically free.
