I sometimes have a DataFrame with thirty or so rows. How can I show all rows? ?show,DataFrame-method is unhelpful.

You can control the number of head and tail rows to display via global options showHeadLines and showTailLines. So for example in your case, you can do options(showHeadLines=50).

This is now documented in S4Vectors 0.40.2 (BioC 3.18) and 0.41.2 (BioC 3.19).