MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
I've never had an issue with belt tension. For Z, it's easy to slide the Z motor to adjust if it is an issue. For the carriage, I used the tensioners related to the Core-XY design of which this is a derivative.
It can be expanded. People have done 1m x 1m x 1m printers (e.g., http://www.thingiverse.com/unknown/about ). However, they invariably need to use four Z leadscrews rather than three for the build platform. I do not know at what volume they find that necessary. Also, they have to stiffen the X and Y linear supports to deal with sagging over those spans.