Geoffrey Young gave a cool talk on this at ApacheCon this week, going over his frustration in moving from mod_perl 1.x to 2.x and how much he liked it once he understood the reasoning. While it's a bit harder to follow without the audio, the slides do point out several pitfalls.

One other nugget was Apache2::porting, which can help you find the functionality which has moved.

Sidebar: I have in my notes a chunk of Apache2/mod_perl2 code he showed off which handled filtering outbound HTML and wrote next to it: solves the absolute URL problem on perlmonks!. One could just filter the outgoing HTML to strip absolute "http://*perlmonks*/" cruft out of outbound posts.