Thanks for this question. It helped me clarify my own quirky interests in the Nokia N900. IMO, you're not missing anything in a phone "as such" in Android. In fact, the phone/mobile functionality in Android is no doubt better than the N900 (barcode scans, voice dialiing, integration with google voice, phone-related apps, etc.).

My own interest in the Nokia N900 is less as a phone than as a pocket computer. So I think some of these debates --- Android vs. Maemo --- come down to whether one is primarily interested in a phone or a pocketable version of a Linux/Unix desktop. And perhaps if you're not a *nix-head, then Android phones will seem every bit as capable as pocket computers, especially with good Google docs integration. Alas, I happen to be a *nix-head.

As far as I can tell, Google didn't want to create a typical *nix/POSIX userspace because they want very tight integration with their own services. Hence, Android (and the forthcoming Chrome OS) assume that you're not going to want to install a lot of funky local apps, but rather use your phone to access gmail, google talk, and Google docs. Like the iPhone, Android is based on the premise that mobile apps are an entirely different species than desktop apps. Which arguably makes the Android a better smartphone for people who want that. It also makes Android more mainstrem, since the combination of thin client + cloud services Google provides are meant to compete directly with Microsoft's offerings. But all that makes Android less open as a mobile "computing device."

A locked-down environment also helps Google protect the closed parts of the system (e.g., the Google-specific apps), as was evident in the recent legal actions taken against CyogenMod.

I'm sure someone will be happy to correct me if I'm wrong on any of this.