Now, I'm not trying to incite a native vs. web flame war here, but I believe that building separate native UI layers would have actually been much better. I hope I can provide some constructive criticism on the native app for the Mac:

The app, as it stands, does not behave like any of the others on my screen. This greatly increases the startup cost for me, since it doesn't follow the idioms I've already learned (and which I'm concurrently employing in these other apps).

For instance, the code editor doesn't behave like any other text editor on my screen (including the Xcode and TextMate source editors open alongside): Ctrl+Arrow doesn't navigate; double-click-and-drag doesn't select by word; my services are not available in the contextual menu; etc.

The keyboard shortcut for "change tab" that works in all the other tabbed windows on my screen does not work in this interface. The same goes for zooming shortcuts, "find" within the source editor, etc.

Then there are more fundamental disconnects: there's a "save" button in the interface, for example. That's an extremely rare thing to see in a Mac app. In fact, as of Lion, the user should never have to explicitly save.

Now, of course, building a native UI layer (in terms of design, not just ABI) would be a great deal of work. You could reasonably argue that it wouldn't be worth that work, but I'd just like to point out that it would indeed provide a much better experience.