If you're talking like a die hard WOW main tank/raid leader, no offense, you probably will have some problems. But if you're good at adapting to the situation in front of you there will be tons of fun to be had. Right now most of the "boss" mobs in the game can just about one- or two-shot any of the melee classes as the game requires to be fast on your feet and just get the fuck outta the way, rather than standing there taking the hit. That being said some of the classes do have limited options to mitigate some damage.
I didn't try out any of the Dungeons, but I was able to successfully take and hold aggro on my Ranger during most of the Event Bosses. The range really helped out a lot and being able to switch between weapons on the fly (most classes can do this) opened up a lot of different options for CC and DMG alike to keep the boss under control. I was also fairly successful at tanking on my Elementalist using a staff, this is the only class that cant switch weapons on the fly but they do instead switch elements, but I was definitely more successful on my Ranger. I also never tried any of the other weapon combos on the Elementalist.
My Thief on the other hand was TONS of fun and has a ton of abilities to get the fuck outta the way, but one hit and he was toast. I had virtually no success in tanking on him and avoided doing so at all costs. But with the proper weapon combos the Warrior and the Guardian should have much more success at melee tanking. Although I did see a bunch of Warriors using bows in order to tank, so who knows.....