No it isn't. Extremely easy to keep track of which one you're targeting. Call Target the one you want your team to focus, and you never lose track of him.
Want to focus the Mesmer? It's the one with sparkly pink/purple animations who is doing more than just auto attack.
Want to focus the Thief? It's the one in medium armor obviously, but to distinguish it from Engi or Ranger, it's the one with shadowy/bloody/acrobatic spell animations. He might also be sporadically disappearing from sight and shoving a dagger in your backside.
Want to focus the Ele? It's the one with glowy magic (water, earth, fire, or air) about his wrists, and obviously throwing out one of those four elements.
Distinguishing one class from another is as easy as seeing a single spell animation, (as every single profession has unique themes to their animations, even most of the auto attacks), and actually knowing, in general, what their animations are. It's a bit more technical than LoL, because you have to actually remember what skills do and look like, but that just means it requires more skill than the average low-elo player in LoL can handle.
Beyond that? Tab tab tab tab "Oh, Mesmer icon - CTRL + T KILL IT!"
Whether or not you want to kill the Mesmer, Thief, or Elementalist first, btw, is not simply a priority of which profession is actually more dangerous. You have to figure out how each player is building without being able to see their gear itemization or skill bars, and can really only see what weapons they currently have equipped.
Again, simply more skill than the average low-elo player can handle, as they're trained to distinguish target priority entirely by champion choice. Even if you believe Mesmers are more dangerous, what if their team built with that in mind? So the Mesmer has built up tons of Toughness and Vitality and has heavily defensive traits, expecting you to try to focus him down first... so the Thief and the Elementalist are full-on glass cannons.
Combat in GW2 is going to be more chaotic with more split-second decision making, that could go terribly wrong, than you might expect in other PvP games. Reminds me of how soccer goes, but maybe that's just because I came fresh from the soccer fields, earning my referee cash.