what features/services should I look for when deciding a domain registrar
A good reputation always helps, as do recommendations (presumably one of the reasons you're asking in the first place?!?). Although cost will always be a factor, it's not necessarily the critical one here. Ease of registration, speed and quality of support, ease of transfers in and out, ease of individual and global domain record updates - all items that you may want to factor into your choice of Registrar. BulkRegister have performed brilliantly for the last two years on all of the above - as will have many others I'm sure. Personally though, I can't recommend them highly enough.
R.
They've really cleaned up their act from a couple of years ago, and believe it or not, we've found their customer service to be simply excellent lately.
This from the chief hater of NetSol 4 years ago. :) So I mean it when I say I've been really impressed lately.
They've also got some really cool features for managing thousands of domains.
I have no affiliation with Gandi beyond being a customer.
My bulk domain registrations are with DotRegistrar (requires large qty). For 'no muss, no fuss' - NameBargain is probably the best way to go for small quantity registrations.
I also reluctantly use NetSol for the domains I've been too lazy to switch over.