I had a chance to watch this set again... awesome.
Andre hit a brilliant balance of consistency and aggression. He was not letting Federer breathe, giving him very few balls that sat up. But at the same time, Agassi was also mixing up pace somewhat --- it wasn't like in Australia, where Andre seemed to take a full cut at every single ball. In Australia, Andre wasn't giving Federer a chance to make unforced errors himself. In the USO Final, Andre still played his percentage tennis... but he played it with incredible intensity, and threw in some surprises: approches to net, drop shots, finesse. He seemed to change things up at the right moments, causing surprising errors from Federer.
I guess he just wasn't able to sustain that level. He was hitting too many second serves, and he hit a few lower-quality shots at crucial points that Federer was able to attack. Then Andre lost that balance of aggression and consistency... shots were either errors, or too meek to bother Federer.
I think he still made an awfully impressive effort in that final.