Flash sucks:
http://flashsucks.org/
My personal problems with it:
can't bookmark individual pages inside a flash site
flash scrollbars only sometimes support the scroll wheel
flash scrollbars are often styled such that I can't fakking see them w/out a magnifying glass
flash sites make it so I can't select and copy text
flash is really slow on my older computers, which I still use (thanks to flashblock)
flash stores cookies and other info on filesystem w/out being deleted when I use my browser controls to clear cache/cookies
flash seems to crash and make stuff unstable
flash makes noise when I don't want noise
back/forward buttons don't work properly on flash sites
absolutely FULL of security holes which stupid adobe takes months to patch
... to name a few.
I don't have any problem with it for things like the following:
videos (although it does eat up a lot of cpu -- my p4 3.0 ghz can't show low-res YOUTUBE w/out stuttering)
animations/comics
games (samarost 2, fantastic contraption)
apps that run outside the browser (like balsamiq mockups)
I hate it for:
full-page flash sites
flash-based site navigation (verizonwireless.com)
ads