I have used imagemagick/GD previusly, but there are some hassle involved in setting up this for each project/client (cron job for cleaning up thumbs, space considerations, not all ISP I use for clients have GD installed etc)
I agree that writing an ad-hoc image scaler is simle.
However, writing a quality general one that handles different formats, image quality, aspect ratio, smart cropping etc..is not trivial.
Also, you are going to be limited to the tools for processing images that are out there for the most part, which sadly means ImageMagick and/or GD. But not SO sadly, as both of those (while annoying to deploy, granted...) have decent bindings to everything, support a pretty good selection of images types, and have params for controlling the aspects you mentioned (except maybe smart cropping).
Everything else is arithmetic and designing a clean, RESTful URL scheme for invoking it. I guess I meant trivial in the sense that all these things are solved problems or only require a bit of time/cleverness.
I built one of these a few years ago using PHP/GD, and it performed admirably (and did everything except smart cropping in your aforementioned list...)
So, feel free to hit me up if you get stuck on any specifics. Been there done that, and can commisserate if nothing else. ;)