Sadly, the development environment matters quite a bit.
- I will miss being able to declare classes using a sane syntax (Coffeescript helps in this regard)
- I will miss the optional type annotations, which caught many a bug for me (you can use JSDoc annotations + closure compiler, but only if you like placing a verbose comment in front of every single variable declaration)
- I will miss having an IDE that can actually do code completion (this not possible without type annotations, so it's not coming to Javascript any time soon)
- I will miss interfaces
- I will miss namespaces (there are various ways to hack namespaces into Javascript. They are all letdowns.)
- I will miss color transform filters, drop shadow filters, and blur filters (might become part of CSS if Adobe's proposal gets approval)
- I will miss the (mostly excellent) API documentation (MDN tries its best, but is not even close)
- I will miss masks (of any arbitrary shape)
- I will miss being able to create and dispatch my own events without having to use someone else's event framework
- I will miss having all my resources compiled into one blob and loaded together, rather than having to worry about preloading all of my UI images
- But most of all, I will miss performance. Yeah, Flash is buggy/slow/memory hog blahblahblahblah. But have you tried doing anything graphically complex in HTML5? Holy shit, your processor will scream. Things that Flash used to laugh at will bring Chrome to its knees. Don't get me started with IE.
So no, Flash is not great. It has a ton of warts. But HTML5 is not ready. Not even close. So we're going to have to spend the next ~2-3 years in techno limbo while it catches up. And that makes me sad.
HTML5 is particularly not ready on mobile. For example...don't use gradients, since they kill performance. Oh, and don't use opacity either, since that also kills performance. Seriously? I need to build a UI that doesn't use opacity animations? FFFfffuuuu--
Apologies for sounding like a grumpy old man, but promises of "someone will come and fix it with a standard...at some point" don't really get me excited. I worry that in vilifying all the things that Flash did wrong, the web world is going to ignore all the things it did right, and refuse to integrate those things into HTML.