HTML5 and CSS is still a little hit and miss. For example JS oriented animations can be amazingly slow in Chrome, but if you use CSS transformations instead, they can become smooth as silk. My point is that to build something amazing, you do have to learn how to get the most out of it; what to use where, what works, and what doesn't.
However I have never seen as much development on a standard, as there is for HTML5/CSS/JavaScript right now. If you build a HTML5 site today, it WILL run better, smoother, and to a higher quality in a years time. It's hard to find other platforms, even very mature ones, where you can see half as much development. It's also spreading into so many different areas, being ubiquitous on almost all platforms. It's even a first class citizen for building apps in Windows 8!
It's certainly far easier to build very sophisticated, and pretty applications then using Java/C#/C++/whatever, and that's what I really like about it.