Maemo is still a little "rough around the edges" in comparison to the iPhone and also possibly to Android.
However, the N900 has WAY more potential (even in it's current form).

The place that this is most obvious for me is in the phone functionality. Compared to any of the other recent Nokia phones released in the past 3-5 years the N900 is miles behind (groups, voice dialing, custom ring tones etc).
Many of the apps that are currently in Extras-testing are VERY geeky in nature, but I think over time that will change.

The other thing is, that, as Maemo is such an open platform, there are Community apps appearing in Extras-testing that are designed to overcome some of these shortcomings (and there will be more in the future?)
The cool thing about this is that Nokia can now get real data as to what it is that end users actually want from the download numbers, and then approach the author of that utility about including it in the "base" OS (or at least that is how I would do it).

GAHHH, yet more Americans who don't understand handset pricing. *All* top Smart phones cost around 5-700($/€/£) unlocked when released.
Over here in Europe, you can get the N900 for *Free* (if you are willing to sign up for one of the more expensive plans, i.e. something similar to the iPhone one!)

DISCLAIMER: I have a loan N900 from the Summit and found that it is the perfect device for me as it is a "mobile Linux computer, with a built in phone".