Unwarranted applause is annoying, but I have been known to do it from time to time. Here are my applause criteria for what it's worth.
1) Clearly tough conditions for landing. I remember traveling in Argentina and flying AEP
-BRC--after a smooth flight we had to fly what seemed to be an unusually long approach--and then I realized that the area was subject to really severe crosswinds--which would sudenly gust and literally move a plane 5 yards laterally off course! The pilot (as it turned out a pretty old guy) flew the 732 manually working the engines in response to each gust. My wife was terrified, all I could do was to tell her to trust the pilot... A few people did throw up, but had enough strength to deservedly clap on touchdown.
2) A very long flight. If you flew from Singapore to NYC in economy, you might as well clap and celebrate the fact the whole thing is over.
3) Unusually smooth landing (like one poster says when you do not notice you are on the ground)
4) Dealing well with emergencies of any kind