One thing that has surprised me with web fonts is that there seem to be plenty of respectable choices available for free from the likes of Google Fonts and Font Squirrel, and relatively few fonts I would even consider for serious work from commercial font-as-a-service shops like Typekit and Cloud.Typography. After years of me telling people that professional quality results need professional quality fonts, the industry seems to be doing its best to prove me wrong.
I suspect the main difference is that the good freebies were designed for on-screen use from the start, while many of the commercial ones are adapted from existing print fonts already available from the likes of Adobe and Hoefler & Co. Unfortunately, many of those designs simply don't adapt well to the lower resolutions and anti-aliasing used for on-screen rendering.
The trouble is, I don't really care why a font on my visitor's screen doesn't look good, and neither do they. The only thing that matters is that it doesn't look good, and in many cases that seems likely to remain the case until much higher resolutions become the norm for all screen sizes. Until then, as demonstrations like the one we're talking about show very well, there are plenty of decent screen-optimised fonts available from other sources, and they don't come with the financial and legal headaches of the rent-a-font services.
The web design industry, fad-driven as it so often is, seems to be in collective denial about this. It's like everyone waited so long to get some variety into their sites that now they'll use anything that isn't one of the handful of long-standing, widely available screen fonts. Unfortunately, being different does not necessarily mean being better.
(Edit: Just to be clear, the above isn't intended as a criticism of all web fonts, nor any reflection of the fonts shown in the linked examples, several of which render very nicely and show the technology of web fonts to much better effect. I just thought ogig's point that many web fonts render poorly on some OS/browser combinations was a fair one.)