Hi Justin,
Thanks for your feedback!
1. Polygonal selection - We'll add this to the list, but it will be lowish priority since we already have rectangular and lasso selection tools.
2. Invert selection - This is pretty easy, we'll try to get it in one of the next couple of releases.
3. Navigation while selecting - You can do this by pressing "c" (for camera) to move the scan, then "r" for rectangular selection, "s" for lasso selection.
4. Section Analysis - This is not likely to happen any time soon, but you can achieve this by duplicating the scan you want to isolate, and then work on the duplication (and remember you can undo with ctrl-z if you make a mistake), then use the duplicate scan instead of the original in your mesh.
5. Noise reduction - We're releasing a smoothing tool in the scans screen that can be applied to the scans, or to post-merge mesh. Regarding the noise reduction in the scan setup, we actually don't have the documention on that in there yet, we'll get that sorted out.
By the way, we just started a promotion where every month we're offering a chance to win one of three $25 Amazon gift cards for reviews on our website. Since you sound happy with your THREE, it would be awesome if you left a review. Right now there are only two new reviews this month so at least at the moment there's a really good chance of winning. :)
Trevor.