The missing USB OTG / USB Host is a limit. Maybe the USB Host will eventually work thanks to some good coders, but even then it may never leave BETA Stage meaning that not everything may work.
A Compass? Well, i never feeled like needing a compass in a device like the N900.
The N900 doesnt have a compass, but the GPS seems to be very good. Maybe its due to the AGPS support, but its faster than the GPS of my Car. The (free) software isnt that good, but thats not a hardware limitation.
A Deathray? Well, that could come in handy, but the iPhone or the xPeria doesnt have one too... and the GPLv3 doesnt allow Deathrays because they are evil (the GPLv2 does, i dont know which of them applies for the N900). I would expect Microsoft to be the First one offering Deathrays or anything like that. Maybe Win8 will have built-in Deathray support.
The iPhone 4 has experimental iReality. Its a reality-distortion-field emitter that makes iPhone users believe that a Mouse with two buttons or more is actually, uh, mighty... and stuff like that.

bear in mind, a lot of those 'x' will not run 'y' because of 'hardware limitations' speculations can be chalked up to corporations fooling mass market buyers into buying the next model of the same thing (a la iphone). 'revamped' hardware 'redesigns' and 'software optimizations' = OS 'refinement' to run 'better' on the 'same hardware'. true story.

Correct. No xG data so far on my device. I am testing the device once in a while for the future wild travel days...

For T-Mobile USA customers, I would say lack of USB host/OTG and HSPA+ will prove to be limitations compared to other commercially available devices a year or so from now.