Let's talk about this chart for a second:
http://www.google.com//finance?chdnp=1&chdd=1&chds=1...
Ok, see that 4000% increase in 18 months? That's what a bubble looks like.
Now, please point out to me where on the graph it made more sense to invest your retirement money in the S&P 500 rather than Amazon.
Wait, you mean pets.com, not amazon.com? Fair enough. But now I'm not sure what your argument is. That color.com is going to fail and the investor are going to lose all their money?
That is what you're saying right? Because if you're saying they're taking a large risk that has a very small chance of paying off then you're saying nothing. That's what investors do. That's what makes us (as a society) all rich.
If your argument is that capital is being misallocated then you have to say why and where the capital should be allocated.
Which brings us to what is actually a bubble and not just people with lots of money taking big risks that may not pay off:
1. capital being invested by (otherwise) non-investors
2. who can't afford to lose
3. who have come to believe, with certainty, that they can't lose
That was the case in both the stock bubble and real estate bubble. It's not the case now. I guess the headline "valuations are unrealistically high" wouldn't generate as much heat and would require a coherent defense.