I value:
- whois privacy
- minimal downtime (:p)
- full featured (I don't actually know what I need from a domain registrar, but I don't want to find out after months of usage that I need some feature that my domain registrar doesn't support)
Big registrars can’t afford any support costs since they prefer to squeeze the price down as far as possible, and therefore they prefer to simply lose or outright drop any customer in case of any and all problems. Conversely, small registrars may charge more, but have better (i.e. actually existing, and sometimes even dedicated and personal) support for when things go wrong, and have a vested interest in keeping you as a customer.
A small registrar might also be so small as to know you personally, which will help monumentally against any social engineering attacks.
(I work at a rather small registrar, with lots of small clients, including lots of private individuals, but also some very big customers; i.e. company names you might recognize. We aren’t cheap, but our support is worth it to our customers.)
If you're just using for registrar, I would absolutely use them with no reservations.
A quick look showed their nameservers were returning completely incorrect records for all of our domains (e.g., the A record for each domain resolved to a bunch of different IPs that were not ours and seemed to be other customers').
They had no status page to indicate there was/wasn't any issues. I contacted support and they said it was resolved but wouldn't tell us what or why it happened.
When we started transferring domains out, sales contacted us and asked what was going on. They had no idea what we were talking about when I explained what happened. I asked them if they could provide any more information, or if the company would be publishing an incident report. They contacted some tech teams and said all they could tell us was that it was fixed and wouldn't happen again.
The issue itself wasn't so much of concern as the complete lack of transparency or communication.
No idea if things have gotten better since because there are about a zillion options for registrars and DNS hosting so I've had no reason to go back.