A neat article in PLoS One about peer review: it turns out that your rejection rate as a reviewer is related more to where you publish than your experience as a researcher. And if you publish in higher-quality journals, you're likely to reject more papers.
There are lots of ways to analyze and criticize this result, but here's my take. If you generally publish in higher quality journals, you will be doing most of your reviewing for higher-quality journals, which are more selective.
As I mentioned, the authors didn't find a relationship between experience and rejection rate, but I thought this figure was interesting: early researchers are clustered around the same "rejection intensity", while later ones tend to be spread out between much more stringent and much easier.