The problems with Flash is that it is closed source and is not an open standard.

Why is this bad?

1) It goes against the very nature of an open internet. E.g. html, javascript.

2) If flash is not available on your platform then you become locked-out.

3) Flash does not degrade gracefully. You get a message to download it.

4) Related to point 3 I can still browse the web with a text-based browser.

5) It is controlled by one company. They control its direction not a commitee standard.

6) Any decent tools have to be paid for. Generally barrier to entry is high.