They started with an already well detailed, quite deep, world (including already having various stories), which already had quite a number of fans (thanks to it being in a series of Role Playing Games - which is pretty much the kind of games closest to a TV Series or Movie), didn't fuck it up by writting a bad script, didn't fuck it up by getting bad actors and didn't fuck it up by being cheap on FX.

It's not exactly surprising that it was a success..

It's maybe a little optimist at this point to expect at least 5 seasons - mainly because that's plenty of time to fuck it up, especially around the point when the initial story either wraps up or gets boring, which is were plenty of great TV Series turn to shit - but yeah, so far so good.