WSJ has a discussion between Todd Dagres and David Hornik:
Mr. Dagres begins: Web 2.0 is a bubble for 3 reasons: 1) There is far too much money chasing Web 2.0 deals. Too much money means too many companies getting funded at higher valuations. 2) There are virtually no barriers to entry in Web 2.0 and therefore the ability to develop a unique solution and sustain a competitive advantage is virtually nil. Therefore, it’s difficult for Web 2.0 companies to build long term value. 3) There is very little liquidity in the market for Web 2.0 companies.
Mr. Hornik responds: I do not believe that the existence of too much venture capital money chasing too few interesting ideas constitutes a bubble. The Web 1.0 bubble inflated because the public markets were willing to bet on unproven ideas. Public markets are ill suited to evaluating such risks. On the other hand, the venture capital community exists precisely to take on that risk. While many Web 2.0 companies will fail, they will not likely fail in significantly greater proportions than has been the case with other venture investments historically.