If you want to use CSS3 tools, even those that are widely supported across browsers, you often have to code up the same effect 3/4/5 times with slightly different syntaxes.
If you want to use HTML5 elements, even just the simple semantic ones, you need to incorporate backward compatibility fixes for IE before version 9 (which is a very substantial chunk of the browsing public, since WinXP only goes up to IE8).
If you want to use the more interactive HTML5 elements, it's even worse. HTML5 video won't be a credible successor to Flash video until there is a standardised, high quality a/v format that all browsers support. Even then, the simple fact is that Flash video players offer way more powerful customisation and some features involving content streaming and protection that are still commercially relevant even if the "everything should be free" crowd want to bury their heads in the sand.
We have SVG and HTML5 Canvas technology to do drawing without relying on plug-ins these days, except that when you actually try to use them, you find that they aren't always supported to the same extent on all popular browsers, particularly the mobile ones where avoiding the weight of downloading a plug-in seems most advantageous of all.
Did I mention web fonts? (Not that it matters until the on-demand font services realise they aren't special and most of us are never going to rent a font instead of paying one-time fees like we do for stock images, blog templates, etc. Or until the rendering on some platforms means most of the fonts actually available don't like the rear end of an animal with stomach problems. But hopefully those issues will pass, at which point having a common font format would be handy too.)
All the while, Mozilla and Google are having a pissing match to see who can release some bleeding edge thing faster than the other, but since no-one else supports it the same way yet, it's basically worthless. Meanwhile, they keep pushing releases every few weeks, and they do break things, often those very HTML5 and CSS3 features we're all meant to love that are supposedly driving web technology development. (Google screwed up rounded corners for several successive versions. Firefox still doesn't get basic font rendering right on a lot of platforms, and the current version can't even look up locally installed fonts properly. Don't even start trying to do tricky things like using Java applets -- and before anyone chimes in about obsolete technologies, remember that there was a big push to improve support for applets not so long ago, and that many state-of-the-art programming languages run on the JVM.)
The recurring theme in all of this, of course, is that you can't write one thing that runs reliably everywhere today. Until you can, any so-called standards will just be marketing propaganda for one browser team or another.