The logo is a bit asymmetric: The distances begin-middle and middle-end of the arrow are different, thus the middle is offset to the left, even in spite of the wider letter "M". Also, in my opinion the arrow color is too "boring".
The asymmetry gives the illusion that the arrow is travelling to the right. Although I don't like that kink in the middle. Nevertheless, compare the that logo with the pentagon one, the arrow one feels like its moving, to me at least. The arrow colour can be changed, but as I've said previously we should be talking about the actual design, and not about the colours or shading.
I think Ricky's latest edit makes the power symbol a bit more interesting, but I still think the logo is not that memorable or meaningful. What about a power symbol means "simple" "fast" "media" or "library"? The idea that SFML "gives you power" is a bit far fetched and generic, every library can be said to do that.
But despite all this, I still think we've got better logos than the direct competitor, SDL.
Also, I really love Haxe's logo...