You can see the effects anti-aliasing would have if you did this short experiment:
1. Run your level so that objects with hard edges are seen, such as the edge of a block or a polygon where nothing is behind it but sky or some other object. Run in 1280x960 (not 1280x1024).
2. Take a screenshot (save as BMP or PNG, not JPG - use file_for_screen to do this).
3. Run the level again only at 640x480 resolution and take the screenshot from the exact same position, again using BMP or PNG.
4. Run GIMP and open the screenshot you took running in 1280x960. Go to image > scale image, make sure linear is used for the interpolation and scale the image down to 640x480.
This scaled down image would be the effect produced by 2x anti-aliasing. Compare it to the one without it. The one with 2x anti-aliasing is much clearer and almost seems to lack fine edges and looks more life-like. The one without it has considerable hard edges and doesn't look as good. If you want to go a step further, repeat the process for a 320x240 screenshot of the same area. Scale down the 1280x960 image to 320x240 and the 640x480 image down to the same size. The one that was 1280x960 simulates 4x anti-aliasing where hard edges are virtually undetectable, even with 16x zoom. The one that was 640x480 does have the hard edges noticeable, but they are tricky to see. The one that was 320x240 has serious hard edges and are easily seen, even without zoom. So yes, anit-aliasing makes a huge difference in visual quality, especially just 2x. Much above 4x has little meaning. The mountains' edges in my 2D game have what would be 2x anti-aliasing as there are 5 degrees of transparency (0%, 25%, 50%, 75% and 100%). You can only detect them by running at a low resolution (800x600) or taking a screenshot and zooming in.
I suggest that the default is set to 2x. Why 2x as the default? It takes more graphics processing power to render anything higher and, short of using a rather low resolution (not in windowed mode) and/or taking a screenshot and zooming in, it's barely noticable.
