The N900 has a high-resolution (800x480 on 3,7") and a very accurate, high-precision resistive touchscreen. This makes it possible to draw with fine detail, write, select, or press small buttons on the screen using the stylus of the N900. In addition the fingers can be used for coarse input.

Capacitive touchscreen have much lower precision and cannot be used with a stylus. Therefore all UI elements have to enlarged for unambiguous touch input.

A GUI designed for finger input can display considerably fewer UI elements screen as all elements for user interaction need to be magnified. Many applications for Maemo5 and other mobile OS have optimized their UIs for finger input which often results in a very constrained or deeply nested UI structure. For example, the close button X on the title bar uses a large percentage of screen estate which could have been used for displaying useful application related information on a normal desktop UI. Another bad example is the PC-connectivity manager, which tries to fit the enormous amount of options into multiple tabs, scroll panes and dropdown menus.

However, many applications ported from desktop Linux can only be used with a stylus due to their small UI elements. If they are not running full-screen, the title bar is unnecessarily large for a stylus user.

Some people prefer to use a stylus for every-day use (due to large fingers) or under special circumstances (with gloves, dirty hands, cold wheather) and would prefer to have less screen estate wasted for too large UI elements.

Some applications are so useful that one would like to use them on the desktop as well and it should be possible for the appllcation developer to adjust their UI to desktop screen with minimal changes.

Which solution could satisfy the needs of different user input preferences, application requirements and developers?

Brainstorm and solutions

a related discussion from N800 times

AFAIK Nokia's plan is to replace the resistive screen with a capacitive multitouch screen in future devices and thus to support only applications which have a finger-optimized UIs.
I think that's a very bad idea, but if you like it, feel free to add it as a solution.

Not only is it also possible to implement multi-touch on resistive screens (google Stantum)
but multitouch forces you to use two hands.
With clever placement of zoom (or volume) buttons there is no need to multitouch and
you can easily use the device with one hand.
Just try the browser in portrait mode and zoom with the volume button on the N900 - much better than multitouch, IMHO.