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WEB 2.0 SUMMIT — Yahoo ad-sales executive Dave Karnstedt is very nice, we hear. If by "nice" you mean "ineffective" and "over his head," his sniping critics say. We'll stick with just "nice," though, judging from his demeanor at this conference's online-advertising panel. He's so nice, in fact, that his voice keeps cracking during the panel as he talks about how Yahoo's going to grow traffic on websites it owns. (Funny, I thought traffic was dropping.) Yahoo has, it's true, been able to squeeze more revenue out of existing sites. But without substantial growth in its traffic, it's not clear how Karnstedt is going to make his numbers look, well, "nice."