Up/down voting seems good. I wonder if you might get a bit of an head start if you combine the shorter sentence idea with selection based on higher n-gram coun

Up/down voting seems good. I wonder if you might get a bit of an head start if you combine the shorter sentence idea with selection based on higher n-gram counts. For instance, if the keyword + words either side match a common n-gram, you could expect that sentence was reasonably representative and boost it in the initial rankings as compared to an n-gram that has a much lower count.