Since VisualEditor is active on English Wikipedia now I had experimented with it here and there. And what should I say – I hate it! Whatever I do quickly with a few characters in Wikitext (e.g. Wikilinks, text formatting, embedding graphics, templates, etc.) needs at least the same amount of clicks through various windows in VE and takes ages therefore. In my opinion this is just unacceptable for experienced users – and I'm not the only one as the overwhelming consensus regarding the necessity for an opt-out of VE shows.
I don't want to get into the discussion whether VE is a good or a bad thing here. Personally I even think it's great for newbies or people with a reluctance towards markup in general. But there needs to be a choice!
I therefore strongly suggest that – however the final FLOW will look like – a tiny checkbox is installed somewhere in preferences where one can select whether to edit comments in a simple markup editor or VisualEditor (and maybe even a small button in both editors to switch to the counterpart directly from the UI).
Don't get me wrong, I don't want to be able to use all functionality of Wikitext (Jorm already pointed out that this won't be possible due to how comments will be saved in the backend). But all formatting that will be available to the FLOW-VE editor (which will only be a subset of the normal functionality, too) should be available in a FLOW-Wikitext editor.