Say you're considering purchasing an indie game, in the price range of $1-$5 (not yet determined). This game is a top-down action RPG similar to those you would find on the SNES console. It is for the PC.
Now, one notable feature of this game is that it requires online co-op to finish. Side quests, mini-dungeons, and world-map activities can be done solo, but all primary dungeons involve puzzles that require both players.
Is this acceptable?
As an answer to some people's questions: Yes, you can make public games, and anyone can see them and join them. Private games ask you to set a password, which you would tell to your friend.
Note that for option 4 to work, one or both players would need a gamepad, or both players would have to share a keyboard. Which sounds awful to me, but what do I know?