The biggest difference between registrars is the type of company you deal with. For most people, a domain registrar is just a company through which they create a record in the domain name registry for the TLD they've selected. That means that domain registration services are essentially a commodity in 99% of cases.
However, for whatever reason, the domain name industry has attracted unethical businesses and corruption almost from the very start (look into the history of InterNIC and Network Solutions). And even though practices like frontrunning are technically illegal and supposedly wiped out when domain tasting was eliminated, you're still dealing with potentially sketchy companies in a position of trust.
So it's not so much what benefits a good registrar can provide for you so much as what a bad registrar can do to hurt you. For instance, a lot of registrars are closely associated with domain squatters/speculators (or may even be the same company, as in the case of Godaddy's domain warehousing scandal). You may or may not care if your registrar tells domain squatters or your competitors when your domain is due to expire, or the fact that they hijack expired domains for free advertising, or the fact that companies like Godaddy have long been a safe-haven for spammers, but some of the worst registrars have been implicated in things like credit card fraud and domain theft. The domain name industry is largely self-policing, so is it worth the risk to do business with a shady company?
That said, sleazy business practices are pretty much the norm in the domain name industry. So never be late renewing your domain. At best, you'll have to spend hundreds of dollars to reclaim it, at worst, your registrar could turn it into a neon billboard for your competitors or you could lose the domain altogether.
What I recommend doing is to just take advantage of the free domain that most hosting packages come with. And don't register a domain with a company that charges you for things like private registration. Aside from that, there's not much else to choose between.