I own an N800 tablet (not a N900 phone) and I recently bought a SE Xperia Neo.

When I'm browsing internet I miss my N800:
- Screen was slightly bigger but still pocketable.
- On Xperia Neo you have to use fingers. On N800 you have a stylus, tapping is more accurate, you can use it also wearing gloves and you can always see what are you tapping on.
- I miss hardware buttons of N800 for zooming/scrolling, on Xperia Neo you have to do it by gestures.
- With Xperia Neo most sites [having a mobile version] redirect the default browser to the mobile (restricted) version, even if it would be reasonably capable of showing the full version. However you can install Opera and set it to show the "desktop" version of the site.
- With Xperia Neo you can't use an http proxy without hacks.

Moreover:
- Xperia Neo has built-in DLNA server but doesn't have a built-in DLNA client (you need to search 3rd party software for this). N800 file manager has DLNA client support built-in.
- I can't install OpenVPN without a custom firmware and then voiding the manufacturer's warranty.

As far as I know most of the drawbacks of Xperia Neo listed here are the same of other similar Android device.

On the plus side, Xperia Neo is smaller, thinner, lighter and faster, battery life is good.

On the development side, it should be easy to port standard Linux applications to Maemo but there are probably more developers and applications on Android than on Maemo.