A small and incomplete list of the features:

Support for TIFF, FIT, BMP, AVI (uncompressed) and SER files (thanks to Heiko Wilkens). MOV files and other compressed files are also supported when FFmpeg executable is available to AS!. In principle there are no file size limitations.
Monochrome and color recordings (both raw bayer, and RGB).
Very fast processing due to multicore support and advanced image buffering techniques.
Multiple Alignment Points (MAP) results in accurate alignment especially for wider field images (Sun/Moon), but even planetary targets such as Jupiter benefit significantly from MAP.
Frame viewer to quickly scan through all the frames (sorted by quality)
Sophisticated MAP analysis and recombination to ensure sharp stacking results. Yes, I like to throw in some fancy words, but it actually works…
Automated frame rejection techniques to remove poor quality and incomplete frames.
Batch processing.
And much much more…!

What AS! can’t do: Image sharpening and post-processing

When AutoStakkert! has finished processing, you’ll find yourself with a resulting image – a raw stack – that is not yet ready for publishing. This image contains more detail than think, but you need to bring it out by sharpening your recordings using other image processing software like the GIMP (make sure to get a version that can handle 16-bit data!), or for example Adobe Photoshop. But you can also use other freeware image stacking programs to sharpen your raw stacks, and many people use the Registax wavelets for this!

It might seem that image processing plays only a small part in producing high quality results, but in fact it is as least as important as all the other factors involved in Astrophotography. If the seeing, optical quality, collimation, focus, camera settings, stacking, or processing is less than optimal, you will simply end up with a poor result. This makes astrophotography a challenge, but it also makes it a lot of fun!