I would also add that going forward (so this is speculation, but I’m pretty confident these will be added)
Bike will add rich text support, which will be encoded into Bike files as for example
<strong>my text</strong>. In ideal world I’d also like to encode that in plain text as
**my text**, but it’s pretty likely that won’t happen immediately and maybe not ever. So until that happened then you saved you would loose rich text formatting in plain text document.
I generally expect Bike to encode metadata in HTML structure. So when tags are added to Bike they will likely get encoded as an html attribute. Again it would be possible to separately encode this as plain text, but I again expect that won’t happen immediately if at all.
Maybe summary is… you won’t be losing too much in Bike 1.0, but over time I expect you’ll loose more features over time.
If your goal is cross platform editing of your outlines I think using OPML might be best solution. I expect OPML files should be able to fully save/load Bike features.