While reading a post by Syl, from Raging Monkeys, I found this other interesting post written by Chris of Game by Night. Chris tells about his difficult in playing an asura thief since ArenaNet did such a good job of giving each race a strong sense of culture. Due to that he just finds it hard accepting an asura as anything but as an engineer. With other races he has similar problems but with different professions. I was going to comment about it on his blog but sensing that it would end up turning into a wall of text I thought it might be better to turn it into a blog post instead.
Anyway, I don't know if it is because I have a weird mind or what but I don't find it that difficult to justify any combination of race and class. Making it sound interesting or unique without it feeling convoluted might be hard but simply justifying an off-beat combination of race and profession, not so much.
Let's start with the easy ones, the Sylvari. They are a very young race, being only 20 years old. That is very little time to develop much of a culture of their own or establish strong traditions. They also have a strong sense of curiosity so I can see them trying pretty much any profession as long as it isn't extremely cruel to others.
Next up are the charr. They have the Blood Legion, consisting of front line soldiers, the Ash Legion who prefer the use of subterfuge to get the job done and the Iron Legion who are the ones who come up with all the machinery and guns for the charr. You can choose to be part of any of the three during character creation. The first one is pretty much what you would expect of a a race of big furry, militaristic, cat-like creatures. The other two are more unusual at first but they also follow the same militaristic attitude as the rest of their race. So a charr thief or a charr engineer isn't that unusual. A point that was raised in the comment sections of Game by Night was whether then certain professions would fit with certain legions. I think it is possible and here are some examples off the top of my mind. A thief could be part of the Blood Legion, thinking it would be an easy way to go up through the ranks by manipulating the "dumb" blood legion for his own ends. Perhaps by falsifying reports about his feats that couldn't easily be contested. Something that could be harder to do in the Ash Legion as they probably would be used to that kind of stuff. While a warrior in the Ash Legion could be more of the "street thug" type with the leaders of the Ash Legion using such warriors for when they need to send a clear message to one of the charr enemies but would rather use one of their own instead of asking favors to the Blood Legion. A necromancer could be part of the Iron Legion, studying the ghosts of Ascalon to create better devices so the Legion could get rid of them.
While in the asura everyone is pretty much an engineer from birth, not everyone can simply live off inventing new things all day. Some of them needs to hold what little laws they have or fend off threats to the asura. Those would be the Peacemakers, the guards of asura society. Warriors or guardians could come out of their ranks. If you'd rather not have an asura warrior tied to their military, you could always use their naming culture for it. In asura society anyone who doesn't have double letters in their name are mocked by their peers as they grow up. Perhaps your asura grew so bitter from the experience that the only way they can express themselves would be by extremely violence, preferably with their own patented B.A.S.H.E.R. 9000. While the elementalists and necromancers could be like our real world physicists, people dealing directly with energies or theories that other people would find too ephemeral to think about. Rangers could be the ones who collect live specimen for studying while thieves being the type who prefer to save time by "borrowing" common parts from other krewes to build their own projects.
Norns could work out with different professions too. A thief norn could be a cheater who lies and do anything in his power to make himself look more legendary. In fact, one of the norn character creation choices is of a rival who did cheat against you! A norn engineer is entirely possible too. He could try to be figuring out new weapons to hunt down prey that would be otherwise impossible for a norn. Crafty is something the norns appreciate too and engineering in my view would fall under that. A mesmer could also be a very crafty hunter using his powers of illusion to tricky prey into a trap. An elementalist could be a shaman using his direct line to the spirits to bring the forces of nature to his aid. Necromancers I admit it is a little harder if you don't want to go the cliche of "lost loved one, turned to dark arts to revive him or her." Perhaps they could also be a sort of shaman too but more of an amoral type who instead of serving the spirits seeks way to enslave them to his will.
I am leaving out the humans as I don't think anyone would have any problem matching them with any profession. :p
In the end we always need to keep in mind that in any society there will always be individuals that go against the norm of their culture. Some times it is a harmless thing, other times it ends up in terrible deeds while in other cases it can end up turning into a force for positive change and progress. As pointed out in the comment sections of Chris' post, this is exactly the case for our characters. This may be hard to see given that there are so many other players in the world doing cool stuff too and the NPC population not being nearly as large. But if you consider that the NPCs professions as the norm for Tyria's culture, the merchants, the innkeepers, the soldiers who patrol the roads day and night, etc, our characters are extremely exceptional. Not simply by the circumstances they start in but also because they will eventually be facing dragons. I don't think that is a prospect most of the population of Tyria would look for or be willing to deal with. It is something that will be up to the exceptional citizens of Tyria to do, those being our characters, so that the rest of the world can be safe and continue living their lives as they are used to.