Even a good native GMail app specially for GMail. The default POP/IMAP email client on the other hand is buggy and a bit disappointing.
The GMail app is a Google app and thus only available for Google Android phones, not the N810.

Again, only Android phones get access to the market, but not the N810.

Indeed. Very optimised for finger-usage on a capacitive touch screen.

Not on the N810 since Android on the N810 cannot utilize hardware acceleration for Java or graphics, although the device has the hardware.

No widget support on the N810 because widgets were introduced with Android 1.5, which has not been successfully ported to the N810 yet. Android 1.0 has some widgets (clock, search, picture frame), but you cannot add more.

The Android market is not the only place where you can get apps, so this applies to the N810 as well.

Developing for Android is very easy compared to the N810, but you can't simply port applications with user interfaces as Android uses its own totally different user interface toolkit. The UI has to be written from the ground up for Android. On the N810, X or GTK applications run after compiling without much modifications.