USB host mode would allow you to plug flash drives, usb keyboards and mice, etc into the microUSB port on the n900. Right now the only way one would be able to do this is through bluetooth. I suspect that it's a minor use case for 95% of people.
1. n900 is far more free and hackable than iphone...it's not even a fair contest there.
2. This iphone has tens of thousands of apps, compared to maemo's hundreds. The iphone has many more games as well. However, you have to pay for many of those, and Apple does restrict what you can put on there unless you want to jailbreak it. In the end it mostly matters if it has the applications *you want*.
3. The n900 has far beefier hardware, better camera, etc. It doesn't have all the features that say the n97 has for phone calls (mms, voice dialing). Video calling should be coming soon. I would look at it as a set of trade offs. The n900 won't have *every* feature you could want, but it makes up for it in other areas, like the web browser, the keyboard, the tv out, hackability, so try to figure out if the n900's benefits outweigh the few niggling disadvantages