This is interesting. Based on my own experience, I can tell you that cross-browser compatibility is going to be a PITA. For Wikidata and Ultimate Wiktionary, I'm considering using this library:
http://ianbicking.org/examples/repeat_form/form.html
It's built on top of Mochikit, a fairly powerful JavaScript library.
When dealing with complex forms on top of a relational database (which is essentially what Wikidata is going to be, plus versioning and other wiki-ness), safe failover is really hard to achieve. Essentially, one has to accept that applications of a certain complexity, especially dealing with data entry, require certain capabilities on the client end.
The above forms are one example - where, with JavaScript, you can easily create complex relationships between different elements, without JavaScript, you would have submit the same form 10 or 20 times, use kludges like a fixed number of empty fields, etc. That doesn't even touch upon AJAX stuff like autocompletion, which is hard to do without in some contexts. So when it comes to building Wikidata UIs, I think it makes sense to apply the [[Pareto principle]].
Erik