All of the professions in this game are hit-or-miss, moreso than any MMO I've played. ArenaNet did a decent job of making each profession play and feel uniquely.
My warrior feels like a brawler; all of his attacks hit hard and hit fast but put him squarely into the thick of things (except, of course, the longbow and rifle).
My Guardian feels similar, but whereas I'm rolling everywhere on my Warrior, my Guardian just reaches into a big bag of tricks and blinds, blocks or otherwise avoids attacks. Both are in the thick of things, but I feel like I hit harder on my warrior, but trade that off with a lot more evasion.
My Thief feels like Batman. Watch the target for a second, shadowstep in, stealth, backstab, roll for initiative, dagger throw, Heartseaker, Death Blossom over their heads. Pair it with all the straight up ridiculously evasive skills on a second weapon set? Forget it. I can sit in the middle of a crowd on my warrior and guardian, but my thief feels like she
dances through them.
My Engineer feels like he has a bag of tricks at his disposal. Mines, turrets, tools out the ass. Shoot > blunderbuss > Overcharged shot > net shot feels satisfying, throw a landmine to knock them back when they get close and a flamethrower into the mix and it just feels ridiculously playful and varied.
The other professions, I haven't spent enough time with, but enough to know they feel very differently from the others I've already played.
That's why, when people start playing, if it doesn't click with them immediately it's important to suggest they try a different profession. If I had started with an ele or necromancer, I wouldn't have fallen in love with the game quickly either. Not that they're bad, they're just not my cup of tea. Likewise, my brother started with a necro and hated it, but
loves his ele. My wife started off with a Ranger, then Ele, then got a Mesmer to 80, a Guardian to 80, and promptly fell in love with a thief.
Profession is a very personal thing in GW2, finding one that clicks with you is essential.
In terms of difficulty. Guardian and Thief are of the easier to play classes, where as Elementalist, Engineer, and Mesmer are on the harder end.
Thief can be a little too squishy until you learn to dodge, predict and evade attacks. Starting off as a thief can be a little rough. I agree with Guardian, but I would add Ranger and Warrior to the list of good starting professions as well.
Like I said though, once you get a feel for the game's combat and demands, switching over to any other class is much easier and finding the one that 'clicks' is important. I'll main my Warrior forever, probably, but I'll be damned if switching to my Thief, Engie or Guardian doesn't scratch a few itches my main doesn't.