I don't have the Samsung Galaxy, but I think most of those are general Android questions I can probably answer:

1. Yes, without rooting. There's plenty of apps to do that using Bluetooth (ie: "Bluetooth File Transfer" is my favorite--works for sending and receiving)
2. Yes, with and without rooting. You could use QuickSSHd (based on dropbear) to support scp or SFTP file transfers.
3. What is "Mobile Hotspot?" Is that a tethering app? If so, with rooting, you can (I use "Barnacle Wifi Tether"). Without rooting, it depends on the phone and carrier and whether they want to allow it. Some phones already have it with Android 2.1. Android 2.2 already has it, unless the ROM image you use (or were given by the carrier) had it taken out. I know the stock image for my Motorola Droid has only USB tethering (not wifi), for example.
4. Comparing to the N900, I can't say. Comparing to the N800, it's leaps and bounds beyond vastness better. It's faster, smoother and in some ways better than my desktop experience with the exception of the screen real-estate of my desktop PC. (Well, I'd say having a keyboard too, except I have a folding full-size keyboard that works with my Droid over bluetooth.)
5. Again, I can't compare to an N900 myself. I might as well leave this for him to answer. Near as I can tell, it's very fluid and works very well and comfortably. I'd say it's easily on par or better than the iPhone 3GS in my opinion, but I never lived with an iPhone 3GS for more than minutes at a time to look at it working.

Hope that at least answered some things.