I hate how poorly optimized flash is, and how badly it runs on hardware that quite honestly would have no trouble rendering a similar-looking application if it was designed well. For example, most flash-games or flash-ads run terrible on any machine with less than 1.0Ghz of processing power, when plenty of games designed to actually access hardware in a reasonable fashion will run amazing on even a 200Mhz processor (Starcraft, Unreal Tournament, Age of Empires, Master of Orion, whatever).
Generally, it's not an issue at all because people are using much more powerful computers these days, but some people aren't, and it's very shitty for people to have to deal with something like speedtest.net taking longer to load than to actually do the test. And, unfortunately, most of my relatives or the parents of my friends have shitty computers or they have managed to screw up their computer somehow. Or even worse, they got some 'cheap' one whose motherboard doesn't have a PCI-Express or even an AGP slot. So they can't even remedy the video-card issue for poorly optimized PCs.
I feel the worst experience is when the flash-based ads bog down a browser. If I had a bandwidth based connection, I'd be aggravated since flash ads take longer to load than the actual site I was attempting to view. It can be very frustrating when advertisements with flash have a roll-over sound or a roll-over popup which is very invasive to a browsing experience. Unfortunately, some places I frequent (not here thankfully) have advertisements or other bells and whistles that make the site take a long time to load, or make scrolling down it slow when using an old laptop or an older computer. Again, it doesn't apply to me personally, but some people's displays only go up to 1024x768 still, and they shouldn't be discriminated against.
However, my biggest peeve with flash is that it isn't optional. Silverlight is not much of a substitute, and even if it was, I hate Microsoft. Flash has already been adopted as the industry "standard" and wouldn't be challenged by a superior product. Even if Silverlight was better, almost any streaming-video site, especially Youtube, wouldn't change what they've got to a better product, because what they have works, and they're making money with the system that's currently in place. Same reason we're still using Coal power plants, using gasoline-powered cars, and that DSL internet goes over the same 150-year old copper wire as the land-line in your house.
Also, while I'm on it, Adobe AIR is bullshit. I don't want flash in my browser - why would I want it on my desktop? If you've ever played League of Legends, you'll understand this completely. The launcher is buggy, unresponsive, poorly animated, just like most flash apps.
