I don't have any strict criteria but I'll list off some bullet points for what's going through my mind. None of these are must-have aspects, just some clues.
- Old[ish]
- Chunkier/bulkier than necessary
- Obscure switches
- Requiring restoration and/or conversion work
- Interesting/nice keycaps
- Researchable (e.g. scanned manuals for the computer available online)
- Well made (I can flex a lot on this since so many interesting boards are cheapos... but nothing beats a tank keyboard)
- Unnecessarily complicated and/or over-engineered
- Savable (I'd like to avoid anything that is too far beyond repair)