Hey Alex,
Was just thinking about a "HOT" type category or rating. This way you still have the calculation based on lifetime votes (since people usually install a module and forget about it, and have a HOT icon or something for a module that's getting a lot of recent attention.
I know my own modules that I've installed have been working for years without much tinkering around, so while I may have voted for a module a few months ago, it doesn't mean that the product is not as good now as it was then.
It's going to be a tough one to figure out how to rate it. There's definitely more weight given to a module that has 4.5 stars and 10 reviews, than there is a module with 5 stars and only 1 vote. How to factor that in however, that's your end
I've no idea how to give "X" weight to something. Maybe someone else here has some ideas on it?