Creating maps before they are shown for the first time does not work on Maemo. Currently this leads to a crash at the first start, when the Atlas map is trying to be created. A workaround is described in the development environment page.
Fonts in the control widgets are too big, fonts in the map too small.
A stripped down version of the control box would help. The tabs itself take more than half of its space.
The main menu is currently stripped down and provides only basic actions. Other useful actions need to be added or replacement actions be implemented (e.g. plugin selection).
Maemo devices have uncommon partition layouts and sizes. In particular, space on / is quite limited, /home has more and /home/user/MyDocs plenty. Cached data could be written to /home/user/MyDocs/marble. See also XDG Base Directory Specification and Bug 230714.
I have not tested libgps support on Maemo yet.