I played a wizard in D3 btw
I pre-ordered D3 with a friend of mine and we played the hell out of it during the summer, but at that time the game was horrid. Then why did we spend much of the summer playing it? We wanted to get 60$ worth out of it before dumping it. I don't think I've gotten 60$ worth out of it even now. At the time of release, the first problem was the servers and the game was not playable for many hours release day since the game is always online/no offline single player. items were horrid, legendaries were worse than blues.
Pros:
Speaking of skill mechanics, the skill swap system was actually very innovative. I know many people did not like the departure from the skill tree of D2, but personally I felt the way builds worked in D2 was kind of stale. You'd be mindlessly spamming a single synergized skill over and over till the point it became a mindless click fest. The obvious example is the hammerdin. Sure, positioning yourself correctly to get the best out of your hammers takes some skill, but in the end you're still spamming the same skill over and over. This is why I think the D3 skill system was better even if some people want to call it "casual" or "streamlined" or whatever. What the D3 skill system offered was variety. Different elite packs/bosses required different sets of skills to kill efficiently. All you have to do is warp back to town to change skills instantly. It may sound arduous to some people, but I think this adds flavor to the game.
Cons:
First, ridiculous elite pack abilities especially on inferno. Often you'd get something retarded like invincibility, arcane sentry, plague, walls, fast, etc and you're dead instantly.
Second, reliance on the Auction House. I can't stress enough on how much this sucks. For most of Normal and nightmare you're fine and items you pick up/farm are generally decent enough to get you through the game while buying things off of the AH will make it easier. However, you have to spend ridiculous amounts of money on the AH to proceed through later acts of hell or just to touch inferno. Elites have more health(millions like bosses from previous difficulties) and pretty much spawn with more rage inducing ability combinations. Combine this with the fact that people that reached/beat inferno earlier beat it through several broken skills that were nerfed hard in a later patch as well as farming a static resplendant chest in inferno(patched out as well). So basically you have a small group of people controlling all the high level drops and there are no good ways to farm for good items. I remember being stuck on the end of act 1/second chapter act 2 for weeks because the prices for good items were so expensive combined with the fact that 99% of all drops you get were either absolutely horrid or did not have stats for your class(this was before the buff to drops and allowing item level 63 to be dropped in early act 1). The drop situation when I quit D3 was somewhat better. 75% of rares found were horrid, 25% are decent enough to be sold on the AH. This was with like 204% total magic find including 5 stacks of NV.
Third, nothing to do end game. The bosses on inferno did not drop boss specific items or have higher chances for rare drops. I vaguely remember one of the developers saying that they want to move away from farming items off of bosses(don't quote me on this), so end game became stacking magic find and killing elite/champion packs to get Nephaelem Valor stacks, which guaranteed you'd get at least 1 rare drop but most of the drops were bad.
When you think about it, they really killed end game since the end game of any diablo game was pretty much farm bosses for uniques/runewords/etc and pvp. Well, when I played D3 PvP was not available, 99% of legendaries sucked, 99% of rares sucked, so it became boring very quickly