I found the difficulty to be very much like Risen. If you know what ur doing, the game can get very easy, but if your not familiar with the games secrets\ mechanics, it can be pretty darn hard. Risen however is a bit more balanced, D2 has by far the strangest balance I've seen lately, which makes it hard during some levels (you can't avoid it, in some early levels\ sections the game can be pretty hard), but it can also get very easy after those parts.
The class system is a lot like Fabels "open jack of all trades" one, which I simply love. Combining the trees is ultimately the best way to go, choosing the right skills that will support ur other skills and judging how useful they are. But you don't have to combine if you dont want to, theres nothing wrong with pure mage\ warrior (don't try pure priest thou, and be warned pure ranger might be a bit harder but it depends). The game might be a bit tough if your not going to read some basic skill explanations in a guide or something, but you'll probably do fine, remember just that the game is hard generally.
If you go pure warrior nothing bad will happen. Game will be hard at first and during some sections (unavoidable unless you know where you should train first), but ultimately you will get op at some point and should never have a real problem fighting off enemies. IMHO A warrior-mage combinatin however is actually the best one , this is the one I do on my current playtrough, but it might just be the fact that I'm more experience with the game and know how to do things.
NOTE: Explosive arrow is perhaps the only good skill in the ranger tree, but it might very well be one of the best in the game, so don't be affraid to go this path. As a warrior you'll get thousand strikes, which is also very good, but not an aoe, has a longer CD, and eats A LOT of MP... I recommend reading about the game skills in a FAQ like this one http://www.gamefaqs.com/computer/doswin/file/952260/58701
if you care about this stuff, but even if you don't nothing bad will happened. Maybe a bit frustration
You can actually reassign your talents later on in the game, so don't worry about it to much.