One of my pet peeves is software the "yanks the rug" out from under you. There seems to be a lot of this going around, mainly in the commercial world. What do I mean?
You spend a lot of time designing and developing GIS systems, only to have one of the key features you use "disappear" or radically change in the next version of the software. Often the response is "simply convert to xyz" or something similar. Unfortunately this is easier said than done. For projects dealing in gigabytes or terabytes of processed data, starting over is not an option.
Does this happen in the Open Source world? Probably, however I think that the community support in the OS world is far superior to that in the commerical software world. Paying for support is one thing, having features you absolutely need removed without warning is another.