What make user fun when it comes to combining mechanism?

There are many RPG game which let user combine skills, attribute, upgrade, addon,,, etc.

Actually, it is essential elements nowadays.

I name it as 'Combining mechanism'.

But user's evaluation about those game's specific mechanics, is different as each games.

Some gain praise, some don't.

What is standard? User don't know themselves well but just they feel it is fun or not or so so.

What elements decide it? and how much?

I think if I know it thoroughly and manipulate it, my design power will increase.


Here is my opinion.

First, I feel there exist quantity standard of complexity.
If that is too low, user become boring. If too much, user become headache.
This is common sense, so no help.

Then what?

I think user will become more fun when he feel there are numerous possibility of combination. In this case, more possibility is virtue.

And that enormous possibility's result should be feel to user as concrete, not abstract.
In other words, he should see the difference with his eyes.

Most gamer says Diablo2 is more fun than Diablo3. Why? I think it is because D3's too restricted user's skill choice to 6 slots, and deleted stat point system.
And skill, rune combination system is boring because there is actually few combination that is more good than others in most cases.

So I know here that Diablo game itself's design fail is, challenges to gamer is too simple, boring. There exist just more strong(more stats) enemy. So user need to be just strong. More damage, more defense. That's all.

He don't need to change his skillset, rune at all actually after when he gots all skill, runes.

Because there are just few (2~3) combination that really works, actually it become narrows quickly to just 1, most efficient combination which works well in most(every) situations in game.

Then user become don't need to think, don't need to change anything, just do again and again using same skills between 6 skills, then he become boil down.

The fact that enemy, environment's confrontation to user is boring is same at D2's case, but D2 has advantage that user can customize his character more diversely than D3.

So D3 become miserably fails.

Any comments or more insights?