You're missing quite a bit.
Memory:
Aston uses more than 2 megabytes of RAM. When I ran it it was using around 10 megs. It does use less but it's not nearly as much of a difference as you say.
Stability:
Both are "stable" but Aston themes are fairly resolution dependent. So if you change resolutions a lot, Aston may not be very suitable.
Taskbar:
Aston has not system tray filtering either. Its taskbar doesn't support ActiveX controls. It's Start menus don't support drag and drop. And the taskbar in Explorer is skinned by programs such as WindowBlinds so you have a lot of control how they look and how much space they use.
Aston also does not have any sort of most frequently used program functionality. It has recent but no MFU like the Windows start menu has.
Toolbars:
Explorer allows the creation of toolbars. Just right click on the STart bar, choose "Toolbars" and then create another one which you can then dock to any edge and accepts drag and drop.
Plugins:
Explorer supports industry standard ActiveX controls. Virtual desktops, media players, battery meters, and many other things can be put into a Windows toolbar (Start bar or one that you create).
Appearance:
As you mention, Explorer requires a third party program to change its appearance. There are approximately 10X more "visual styles" / skins available for those third party Explorer enhancements than there are Aston themes.
That isn't to say that I think Aston isn't a good shell. I do think it is. I just think that you're not as familiar with Explorer as you should be to make this comparison. Some of the things these shells lack aren't trivial things. MFU and taskbar grouping, to name two, are big deals.