I'm on the PDLI external reviewers committee this year. I've never published a paper at PLDI but I have attended. This is also my first time reviewing papers. I thought the quality of the papers I was given to review was quite high. They were also on a surprisingly broad and interesting range of topics. Unfortunately there was a inverse correlation between paper quality and how interesting I found the topic. In other words, the more radical and creative papers were in general much less well executed than the ones with more incremental results. Surprisingly, about 30% of the papers I read had no evaluation at all! I think that there are lots of valid kinds of evidence for the correctness of a result, including proofs, implementation, case studies, even subjective critique and commentary. But if the authors don't make any attempt to evaluate or critique their work, I don't see any way that a paper can be accepted today. Yes, I've published papers in the past that have little or no evaluation, but things have changed.