Ironic, being as the studio that made Guild Wars was founded by people who left Blizzard 😛 Playing GW2 right now, and the similarities are even more apparent. Not to mention the classes...Demon Hunter is a combo of Thief and Engineer, with some Ranger-ness thrown in, Wizard is really more of a Mesmer than anything else...
The skill system is something I very much like, as I like having to think out my build beforehand, and even though in Diablo you can switch skills whenever you want you then have a cooldown on the changed ones, so swapping mid-fight isn't going to help you much.
I'd rather there be a skillpoint system, though; instead of learning skills and runes automatically, you get points as you level up and then can spend those on what you want/need. Just gaining skills as you level feels a bit too dumbed down and simplified for me.