Not to forget of course the problem of accessibility and longevity. If digital editions are indeed defined by the fact that they exploit the possibilities of digital media, then it is problematic to say the least that we have not really solved the issue that our current digital editions are very much tied to particular versions of software/operating systems/browers/devices.
It is not enough that we can say that the underlying data is safe because it is encoded in formats that will last (like XML TEI), because I don’t think we can say the same for the functionality which we build on top of our data. Unless one has both the technical skill and the resources to keep updating the interface and architecture of one’s online editions there will come a point in the not so distant future when things will stop working and all we are left with are the data.
I'm tempted to answer that the data *is* the edition, and that the presentation software's task is to "extract" it from the data so that the user can view it, but of course that's only a technicality and you're right: in the end, how the data is presented to the user is what matters, and of course if that isn't possible anymore from the user's point of view there's no edition at all (and I wouldn't really disagree).
Fortunately there's a push to use open standards not only in creating the edition data (TEI XML), but also in visualizing it: the fact that almost all new digital editions are 100% web-based makes it sort of inevitable, although there are catches. I will be attending this workshop next month:
http://easytools.huygens.knaw.nl/?page_id=18
and one of the points I'll try to make is that we have several layers of software involved when creating an edition browsing tool: open standards like (X)HTML+CSS, which might be considered as safe as TEI XML when it comes to longevity; open source "de facto" standards such as the jQuery library, which are so widespread that they will forcefully retain compatibility for the foreseeable future; open source software like eXist; and a lively, somewhat wild open source small programs developed on the basis of the above (f.i. jQuery plugins). In my own edition viewing software I'm trying[1] to stay on top of this stack as much as possible, and going "down" only when absolutely necessary (f.i. using a plugin when the jQuery library doesn't allow a specific functionality). It's a tricky balance you want to have between standard compliance and sophisticated features, some of the latter being expected in a modern digital edition.
R
[1] More precisely, the talented young developers doing the actual development are trying to do that, since I can barely program my TV recorder XD