That's normal, one uses 24-bit palette, the other 16-bit so you'll of course have visible blocks with a lesser number of colors. But you'll have the same view on all hand-held devices featuring LCD screens out there.

However, try to snap a photo with a high-quality camera if you want to actually see the real quality of the screen. Or lower color depth to 16-bit on your desktop machine and then try to compare the screens. Or compare the N900's screen with some other cell-phone/MID device with an LCD screen and you'll see how good actually is the N900's screen.

Grabbing a snapshot DOES NOT show the screen quality, but a framebuffer that is yet to be sent to the screen via the graphics card.