BAD : I think that people in common have good sides and bad sides.
GOOD : I think that people in general have good sides and bad sides.
If you have the same background, interests, tastes etc as someone, the two of you have a lot in common : 'I'm sure the marriage won't last. They've got nothing in common.'
When you mean that something happens or is true 'in most situations', use in general : 'In general, parents care more about their children's health than about their own.' 'Students in general have very little money to spend on luxuries.'
BAD : There are so many things in common between us.
GOOD : We have so many things in common.
Two or more people have (got) something in common : 'We've moved in the same circles over the last ten years and so we have a great deal in common.'
BAD : In Spain it is common that people turn up at your house without warning.
GOOD : In Spain it is common for people to turn up at your house without warning.
it is common + for sb to do sth (NOT that ): 'It's quite common for new fathers to feel jealous for the first few weeks.'