I played the first one equally by myself and with people I met online.Anybody strictly playing by themselves
What I didn't like about the first one was the need to have people with you to do certain levels.
The way Diablo 3 handles level progression, item drops and management, is FAR better than Destiny.
Destiny should scale the game according to however many members you have in a party instead of forcing you to have X amount of people to do which ever level.
Brehs that have played Destiny more than can correct me if I'm wrong, but as far as I remember, it is impossible to complete all of Desitiny's content by yourself.