It's actually a pretty good game now with all the patches and features they've added, but the problem is that it could have been
so much more. The core issue, which IMO is not going to be something the expansion will be able to fix, is that the items fucking suck. Items are dependent on your main stat, so you just aim for stat+vita+all res on every piece with a few niggles here and there and then call it a day.
This will never be fixed because it's the entire foundation of the item system. They would have to completely revamp it for the expansion for it to come near D2's item system, which would pretty much make it a different game.
Item systems and mechanic in D3:
* your damage output is solely dependent on your weapon's DPS. This was a critical design error that was done to streamline the game but in turn made customization a complete joke.
* no charms
* no jewels
* followers/mercs are shit and can't even put armor on them like in D2, and they're not in multi-player games
* no runewords
* crafting is terrible and has really no use at all, the results are low end garbage. Compare this with d2 and the fun and weird shit you could craft (i.e. the "blood" craft tier) that could turn out godly with lucky rolls
* item affixes blow
* no upgrading items, this was a huge mechanic in d2... you could find a lvl 20 rare axe in hell diff with great stats and upgrade it twice to a berserker and have a godly item, you could do this with armor items too.
* no ethereal items, no items with multiple sockets (cept an OP bow Manticore), in d2 you had up to 6 sockets on shit, goddamn
* gems blow in d3 and have a fraction of the uses d2 gems had
* no skills on weapons (whirlwind assassin anyone?)
* low level uniques and sets that completely suck. In d2 some of the best uniques/sets were super low level. Even the godliest of end-game equips would have items like magefists, bloodfists (this is like a level 5 item lol), angelic set, just take a stroll down memory lane and look at this:
http://classic.battle.net/diablo2exp/items/uniques.shtml
I could go on for a while with this, but you'll notice similar traits in each bullet-point above... they fucking streamlined everything. They took a shit on customization. Instead of a real sequel we got Babby's First Diablo.
There were of course things D3 did exceptionally well, mainly the combat and actual gameplay... easily the best and most satisfying in any loot genre game. Hell I don't even mind the lack of skill points. The core issue is the loot... the one core value of the game Blizzard
had to get right they fucked up. That's the gist of it.