IMO, any DVD you think you'll watch at least once a year is worth keeping. Any DVD you don't think you'll be watching again, or at least not for a long time, it's probably not worth it to hang on to it. Donating them is a great idea, and if you go that route, it should be to whatever charity you prefer, not something decided by a vote in your lj!

Another option for you, if rather than reducing your total DVDs you'd rather replace them with others you haven't seen yet, is Swap A DVD, where you get points for mailing DVDs you are finished with to other people, then you can use those points to get DVDs that other people are finished with.