I could list some significant areas that make Diablo 3 a terrible game in comparison to D2, but there is no point trying to change people's minds who like the game.
The easiest way to lay a summation on it is to say D3 is much simpler and lacks a lot of the depth D2 had. If you've bought into the marketing BS and
excuses on why item design, character customization and the way those work together have been streamlined for the better, there will be no convincing someone.
D2 was a much better design and more deep experience,
if you played it beyond being a casual and weren't the type to just do what someone else was doing and make yourself a hammerdin.
All D3 has over D2 is better combat and visuals, and why the fuck not, it is 2012. That is the minimum expectation you would have. In terms of depth and replay value, the game is a fucking flop, big time. It's been bred and aimed squarely at the World of Warcraft player base and you can see that in all sorts of facets of the game.
Not that it doesn't make sense. The WoW audience is Blizzard's cash cow and it is sound business sense to have simplified the game and aimed it squarely at them. It is still the death and departure of the franchise. I lay most of the blame squarely on the game having been made by new people who did not actually create this franchise.
I gave the game a chance, played through to Act 4 Inferno in what was an unhealthy quick amount of time and just shelved the turd. It is totally lacking the replay value and quality of the item hunt that Diablo 2 did. Why the fuck will I keep playing a game to 'upgrade' to a weapon that does nothing but give me 50 more dps and a few more points in my primary stats. What is that ? It's like the gear grind in World of Warcraft, which I did in that game until they released wrath of the lich king and dumbed that game down as well. Never mind that Inferno plays like Super Mario Brothers and kiting, slow-paced combat. So much for fast paced hack n slash arpg. Terrible game.
If you played Diablo 2, just take a loot at the Arreat summit,
http://classic.battle.net/diablo2exp/items/, item design in that game was incredible and added so much depth to what you could do. You could create a build entirely around a single item. That is not happening in Diablo 3. Every class can use essentially the same weapon so long as it as their primary stat. The game is designed just like WoW.
lol, fuck Blizzard. Diablo 3 is a fucking joke, it feels like Call of Duty gone Diablo.