I want to highly commend RootsMagic for this document. It's what I've been looking for, a nice summary and listing of the differences in data handling between Ancestry.com and RootsMagic. I can't stress enough how important a document like this is for the future, and I hope that the RootsMagic company will consider adding versioning to this and keep it up to date, as new discoveries of compatibility issues arise, and as development changes happen, both on the Ancestry.com side and on the RootsMagic side.
I believe that in the future data portability is going to become increasingly important, partly because of the volatility of current genealogical tools and web sites, and partly as DNA usefulness and general helpfulness drives more and more users to permanent and public world trees. Users want to make sure that all their work is completely safe, that if they lose a tool or web resource, they can still move their data somewhere else, with only changes in methodology, not loss of data. I believe RootsMagic is becoming well positioned for this future, a safe data conduit between tools and web sites and tree storage places.
If not already available somewhere, I would like to see similar documents for My Heritage, and others too when they have time (Family Tree Maker, PAF, TMG, geni.com, FindMyPast, etc). This could help users understand what is a safe core of features and data fields to use, safely portable between sites and tools. In the future, users are going to be dropping features and fields that are unique to specific tools. But I'm sure there will always be users who don't want to share, just keep a personal tree, and accept the limitations and inaccuracies inherent.