Every profession can be played "upbeat" and requires lots of switching along with attention, so you'll get different people suggesting different things to you, and likely, you will like them all once you can get into their mechanism.
Personally, for "versatility within combat" I'd say elementalist. Other than the fact that they can't switch weapons within combat, they can switch between direct damage, condition damage, knockdown/knockback/stun, crowd control conditions, healing/support, self survival, or even running away fast any second in the fight. But once you've chose an attunement, you need to stay in that "mode" for about 10 second, which I think is what most people meant by "versatility" --- Been able to adapt to a different role on the fly, which does NOT mean able to do all things at all times.
Engineers are very versatile as well, but it depends on which kit you're equipping, hence sometimes you need at least a break between combat to adjust your kit. I actually think that an engineer can "focus" more than an elementalist in combat, due to the fact that kit swap has no CD while attunement swap does. This means that for an engineer, everything can be optimized in the tightest fashion possible, for example spamming grenade and switching back to rifle for #3 + #5 and switch to grenade again immediately, resulting in higher dps. Engineers do not HAVE to put the kit in the utility slot for "situations", instead they can do so as an "optimization", and doing so has high payback. The problem is, if you're doing that, you obviously can't just be "ready to adapt to all situations" in one single combat. So I say elementalist win out for versatility within combat, but engineer can probably choose more playstyles once you have the chance to re-arrange your kits outside of combat.
The mesmer is "versatile" on a "grand scale", for example portal, feedback, nullfield, time warp, mass invisibility, these powerful but often situational skills require to be switched on and off of your utility bar depending on what your team needs. In combat, I wouldn't say mesmer is "versatile", it is definitely very "dynamic" and very "unique" but since often you don't have control over what happens, I wouldn't call that "versatile". You need to focus a lot on what's going on and often it's more of an reaction than an action to optimize dps/survival within each battle, even though those are YOUR illusions and those are YOUR spells.
And thief, since someone mentioned it, is versatile simply due to the initiative system, which means no CD. You can spend that initiative on whatever you'd like depending on the situation, be it running away, supporting allies, bursting the enemy, applying conditions, ect. Both the thief and mesmer play a bit like "opportunists" whereas the elementalist and engineer often feel like "trying to control the battle field", more so for elementalist and depends on the playstyle of the engineer.
These four professions also have the other qualities that you've mentioned: Elementalist is obviously very magic-y, along with mesmer, they're "casters". Mesmer and thief both have stealth. Thief can feel a bit technology-ish especially with double pistol, and Engineer is of course king of technology.
Edited by CepaCepa, 12 December 2012 - 08:27 PM.