I have the impression that in general, when Beit Hillel and Beit Shammai disagreed, the former was more lenient than the other. But it's not always the case; the g'mara on Rosh Hashana 14b talks about how sometimes Hillel is more stringent, and goes on to say that if you're going to follow Hillel do so for both stringencies and leniencies (likewise for Shammai), and that one who follows the stringencies of both is a fool.
This got me wondering: about what is Hillel more stringent? I'm not necessarily looking for a complete list (which would probably be hard to compile), but are there are particularly-significant cases, or are there specific areas of halacha where Hillel tends to be more stringent than Shammai?