Actually, now that I play through it again, I can see one LARGE advantage of the first game. Developer commentary. In the first game, you practically never had a half-level without at least some kind of developer tidbit. In P2, it's a lot like simply replaying the whole thing without saves, commentary nodes are few and far between. Perhaps they just had a lot less innovation and much more game to fill, but I would've appreciated more talk about the level design decisions, some of the scripted sequences, and just about anything, really. What little there is is indeed fun to listen to, mostly, but it's not enough. It's boring to just play through the same levels again after you know the answers.
I noticed that, too. That said, there's two things to remember:
1) When Portal 1 came out, portals were new technology, and there were a heck of a lot of dev commentary nodes about how portals worked - how to simulate infinite depth, how to let you manipulate objects through a portal, how ceiling-to-ceiling transitions used to crash the game, etc. There was very little new added to the portal mechanics in Portal 2, mostly because portals already worked properly in every situation. Hence, nothing new to say!
2) Portal 2 is bigger, and the developers spread the commentary nodes throughout the levels. There's probably less content, but it's difficult to tell because it's spread so thinly across so many levels. Admittedly, it's a pity they didn't spend more time talking
about the levels themselves - half the fun of the commentary is listening to confessions on previous, failed iterations of test chambers and the like - but then again, that would have got repetitive after the tenth "Well, this chamber used to be different, but it sucked, so we changed it."
I think they should have put the "skip to next developer bubble" button back in.
Hmm. I know they put that button in for the Co-op commentary (I think it's... Tab, maybe?). I found it by complete accident, and haven't bothered trying it in single-player.
I could also point out that the dev commentary nodes return to the density of Portal 1's commentary in the co-op sections. Yay for 10 nodes to a section!
Oh, and one last thing while I'm scatterbrained and thinking about the dev commentary. Did anybody else notice how weird depth perception is with those nodes versus carried objects? Seriously, during the section where you're holding Wheatly, try approaching one of the nodes, and see how long it takes before Wheatly clips through the node. It's... disorienting.