I would actually wonder more what is the requirement for something to not be a preview version. I mean I can understand if you dont want to drop hints about new features but the fact that there hasnt been a full version release for like 6 or so versions kinda makes one wonder which version is best to use. I'm STILL using .95 because .96 and .97 are supposedly lacking something that makes them more stable.
The requirements for a program not to be a preview would simply be one that works without any complications or bugs, and well tested for stability, ect. These types of programs are generally thought out long before developement, and consists of a team, with deadlines, ect.
This program is Steven's "pet project". His aim is to make it functional, and add new things all the time. It helps him gain proficiancy in programming, along with experimentation and just having fun. He does his best to debug,and supply a decent interface - though admittingly it's not quite as fun as creating new toys to play with and suprize his admirers with.
There are quite a few things that Steven has to deal with within OS and graphic cards- moreso now than when he started this program, which is why there has been some stability issues. It's pretty much unavoidable when you are "doing it on your own". It's usually the "trial and error" routine.