800x480 is not the same as 480x800.
That, and more. OMAP3, for example. X, for example. Full UNIX stack, for example (as good as it gets with all the embedded versions).
What do you mean? What applications do you fear will be missing on Maemo 5 which Nokia 5800 provides? I'd rather compare it with Nokia N97, iPhoneOS, and Android.
As long as you have e.g. RDP/VNC/NX and WWAN you can even run such applications remotely saving you local resources. But these applications, running local or remote, are not optimized for the screen size...