But doing boss runs is what you did in D2, you did them over and over again. I highly doubt that you did not take part in the rushing to the higher levels, by the tone of your post. So why keep that failed mechanic. Where this game would be a mindless rush to level cap. Then rinse and repeat when you want a new build.
Then you will not go much higher than nightmare. Somewhere around late hell/inferno will be beyond your skills. That part of the game is for the hardcore crowd. So your adventure ends there. From normal to that point is for the casual crowd. Where as the rest of the game is for the hardcore crowd. You have to remember that there will not be any new quests or stories that will be tied to those difficulty levels that does not exist in the earlier levels.
Diablo has always been about the gear. That has never changed, they are just making it more obvious now than before.
So you only say teaming matters because you have to. Due to the fact the game is designed for teaming only. Sorry if you think that then you do not understand the Diablo series. MP has always been an option. Not a must have in order to succeed. That is unless you had a team centric build.
D3 will have more options that what D2 ever hoped to give us. That is a fact, that you are too blind to see. There will be more builds that will be able to solo inferno than what was able to solo hell in D2.