I look for a registrar that has a phone numbeer and 24 hour tech support. If you have an issue and you call tech support but get a recording that says their hours are 9 to 5 - Leave a message, then I would not use them for anything other than cheap domain registration and I would move away from them as soon as possible. If you are having an issue waiting for them to open is not an option in my opinion.
I also look for a registrar that is available round the clock. With the competition that is rife in the domain industry, why would any such company not have 24/7 support?