Refusing to play uncertainly rated players
Tas: I hate that so many people write "no ?" in their game offers. Give them a break so their rank can become solid. In my oppinion it is exciting to play a ? since you don't know what you are going to get and therefore will play more honestly, you migth figth a hard even game, and later find out that the opponent was several stones stronger than you thought.
If the game is too unmatched, hopefully one will resign soon, no harm done. Take another game with a more appropriate handikap.
Let people get a rank. We were all [?]'s once.
ChrisSchack: I usually don't like to play unranked players, but not because of their uncertain strength ... it's more about how my strength varies, I don't want to mess them up!
Tas: Everybody's strenght varies. :)
Anonymous: I thought that's what bots were for?
AnonLinguist: There's also a large chance that this unranked player knows his strength, and is playing you even, despite knowing he is much stronger. This is demotivating to the extreme, since obviously it's impossible to win. Up to 4 stones is close enough that you might have a real chance, but I've encountered bigger gaps. Given that I only rarely manage over 50 points difference in truly even games, four stones is the limit I want to face without a handicap. But an unranked player could be anywhere up to 10d for all I know. If people mention their rank in a comment though, they're usually not lying about it in my experience. It's the ones who don't communicate except for putting down stones that make me feel suspicious.
tapir: You always have the option to resign. If he is that much stronger, you will not lose much in the end. The bigger problem are very weak players who start a rated game and are not willing to resign at all. (Usually you want to play a teaching game here, give advice later on etc. but being forced to give a teaching game like this, you may not have time later and just squeezed in the time for a single game, isn't very convenient. However, pre-game communication can help.)
bugcat: I usually ask my opponent politely if he knows his strength, and if I don't get a response then I cancel the game (which doesn't incur rating change on OGS).