This is gonna be unpopular opinion considering this is diablo 2 mod forum afterall.
1. cLoD (with synergies update) vs diablo3 + RoS expansion : I take diablo 3
2. Reason, mechanically stand points and because d3 is newer engine. (It will be different if you port d2 to new engine)
3. My experience with d2 is limited, aside from low level multiplayer I played during school. I never played any ladder so I never get hyped by those ladder reset seasons thingy. (Even now Im not hyped with every new PoE challenge league )
4. My assumption that everyone have nostalgia with d2 but I think what could made it was interesting are:
a) Some enemies will have immune to your damage, so you need friends to help you out
b) Build enabler gear. Version 1.10 cLoD i think, added tons of RW that give other classes skill, thus you feel like designed a new skill build with specific gear (similar to median's oskill build)
c) Uber bosses, basically something harder than hell. I never experienced it before, but I treat it like median superhard uber which you required coordination with your friends and some ultra loot gears.
d) Trading economy. Again, since Im myself prefer to play solo, I prefer d3 loot system than old d2. But I do remember I tried to collect some of the class specific exceptional and elite sets and I trade with my friend (more like duping). I found median because I was searching for d2 with increased loots since Hell was pretty tough to play alone.
Here my short points why I liked d3 more
1. New engine, smooth gameplay, Casual friendly, loot drop 2.0, unlimited respec and trying all kind of different builds (although it is kinda limited to what set you are using)
2. Grand scaling of difficulty, play at your own pace. No longer tied to 3 difficulty setting and nice rift and bounties system
3. Replaying is more fun, as in leveling up another character with paragons bonus and high powered gems. You no longer to keep low level unique since you can just craft new set instead
4. Seasons and progression tracking, although Im not fan of it. (It should design more dramatic changes like challenge league in PoE). Right now Im only planned to check d3 after 3 seasons or 1 big patch
I have admit d3 have problems regarding story and gameplay mechanic, but if you ask me to choose either cLoD or RoS, I will definitely pick D3.
cLoD skill design now look dated even if I managed to get all gears in that game.
Im more interested to see on how people have more fun with d2 (aside from nostalgia stuff).
Like I was recently play Titan quest again and I realized despite the problem I have with it, I still managed to crank up to 60 hours of game play.
I have to put it off because I prefer to play other game I have in my list than sticking to one game for long time (that is why I kinda hate TSW stuff with endless grinding)
@OP
You do know that most of the bosses already mention in the book right? I need more reason / arguments on how d2 is 3 times better than d3.
Aside from d3 killing important characters from the past games and retconning some of the old stuff, I dont see any much problems with its' story aside from act boss been very noisy bitch .