I want all apps to look and behave the same, to the point where they're all just one app.
Nobody asked for their shopping experience to be different when shopping on Amazon vs eBay vs Craigslist. I see no compelling reason to have different UI for services that are essentially identical.
What if one browser does something others won't, and you can't do it custom, how do you get around it? Wouldn't that put us back to IE6 web?
What if your site really needs something browsers don't have? Will we have sites telling us to install a plugin and put us back to Flash era?
Custom things will be done through a canvas, but that should only be used in last resort.
Or do you want to enforce it somehow?