I agree fully - but just think, what happens to the sites that *don't* do this? What about the ones that *don't* use blinky ads, pop-ups/pop-unders, and all that? AdBlock blocks all ads by default, so even the people who don't do the annoying ads and shady adware schemes get punished. I'm not saying that I blame anyone for using AdBlock - I'm just saying that not everyone trying to make money off of ads is contributing to that mess.
I'd doubt it. AdBlock Plus is well-known and used enough that it'll get picked up.
If you are a business (which you most likely are if you're even considering "cloud storage") then you're probably going to pay for a provider that doesn't have these quotas - either that or you'll figure that a safe data backup is worth the cost.
Your assumption is wrong. There are FSF-endorsed projects which play Flash/Shockwave content. It'll be able to play Youtube videos through that. Plus, if worse comes to worst, you can just download the plugin from Adobe.