There's another fast food chain that's kind of like this: Five Guys Burgers and Fries. Best burgers I've ever had. And they only serve burgers and fries (the occasional drink, maybe, if you're lucky and you ask nice) and that's it. They make everything from scratch.
I think focusing on something simple and making it super-special is a great way to make a business.
I like In-n-Out's approach. They have a simple upfront menu, but a secret menu if you want something different. The mystery aspect creates an in-crowd, who'll be more loyal.
Google and Apple seem to take this approach too. I don't see anyone trying to guess Microsoft's next big thing.
I've found Five Guys to be really inconsistent from store to store... the local one always serves dried out burgers but the DC metro area Five Guys do it right.
Conversely, I don't like chips and salsa with my burrito, I think it is a waste of money, and that's why I avoid Willy's and Moe's and stick with Chipotle.
I go to Qdoba over Chipotle because the chairs/stools at Chipotle are intentionally designed to make you uncomfortable so you don't stay very long.
Baja Fresh was my favorite quick service burrito place, but the local one closed and was replaced by Chipotle. No idea why they closed, they were always packed.
If you like Chipotle, you should find a Pancheros. Pancheros is very similar to Chipotle, but it has much better tortillas, and that makes a huge difference.
Pancheros tortillas are superior (and fresher), however the rest of their ingredients leave a lot to be desired. And there's no fajita burrito option at Pancheros.
I think focusing on something simple and making it super-special is a great way to make a business.