yeah need to root out some weed
and we better need to be specific about the series cuz not all the games are good in a series
1. Does your choices changes story of Diablo?
2. Do you have to care about relation with your party members in Diablo?
3. Can you control your party member in Diablo?
4. Can you pre-define tactics in Diablo?
5. Do you need as much strategy as dragon age origins in Diablo while in combat?
No offense but...
1. Does your choices changes story of Diablo?
Not A prerequisite for an RPG, there are tons of RPGs out there and only a few(mostly action/western RPGs) having that different story path.
2. Do you have to care about relation with your party members in Diablo?
We yes if you ever played it in multi-player each char has set role, paladin for healing amazon for Hit and Run, warrior for taking punishment(tanking) necromancers for support mentioning just a few. Although you can beat it in single player but its lot more fun while playing with your friends
3. Can you control your party member in Diablo?
No We can't, we can't even control our party memebers in MMORPGs, that doesn't mean those games aren't rpgs(Ragnarok, WOW, A3, Eve Online)
4. Can you pre-define tactics in Diablo?
Well yes if you are playing in nightmare mode with a party, while playing alone its just (hit-run-Recover) Repeat
5. Do you need as much strategy as dragon age origins in Diablo while in combat?
Nope that much strategy isn't required while in combat, I must say most of the western RPGs don't need that much strategy as Jrpgs(Sad part is all good jrpgs are on PS and xbox)
Lets Define RPG
1. Should have Level up system.
2. Should have char inventory.
3. Should have Skills/Magic etc.
4. Should have party.
As for hack and Slash: real hack and slash games are like DMC Series/ Onimusha Series, Darksiders, Ninja Gaiden, God of war.
However I agree that Diablo should be in Action RPG.
Just one advice who are looking forward to play Diablo 2, well its not going to impress you much cuz the graphics aren't what you are expecting (Actually its sprite based 2D game just like Age of Empires 2), not much to do just explore and kill. Not very good storyline. All these things are compared to today's RPGs. Diablo is a Legend Same as original starcraft. One who played both the starcafts will admit that starcraft was a way better then SC:Wings of liberty (gameplay wise)