Mine feels fine opening/closing from either side.

Well, You are not alone. My N900 shows the same 2 Scratches on the back of the slider that yours does.. 3 on mine actually, 2 really noticeable ones close together and 1 light one that is next the the "32GB" logo, In addition, I also have 2 scratches on the front on both the left and right sides of the keyboard, (on the little blank areas on either side of the keyboard)

It must be a design flaw, but cosmetic if that. I don't suspect that this will create any functional problems. My slide is still tight with no flop at all and everything still works mint. Even so, Nokia may replace your (Very Expensive) phone if there are any defects in craftsmanship. It should be part of their warranty. Check it out if you are concerned.

Just checked on mine, despite the aforementioned scratches/lines, the slider is smooth and fluid no matter where I try to open or close it. Either your track is dirty, or there is a bur or defect on your slider rail.

Wish you the best of luck!

i have 4 'lines' on the back of mine, doesnt bother me, my slider is good

nop mine is scratched free there. But got a few from the fine sand dust on the screen.

two long outer lines and one shorter left line.
keyboard looks great although i'm using it VERY much for writing sms and doing modding on the go (file editing / coding with vi rockz! )

I'm on my third n900 and all of them have lines on the back. Its not something I would call a scratch. In fact, the line is very faint, there is no paint coming off. I don't think theres anything wrong, its just how it is.

on your THIRD??? WTF

haha the first one had a few dead pixels and for the second one, the internal memory card was corrupted.