The most important thing, technically, is having great developers who ship.
For piths sake, I want to say "Everything else is noise" but that isn't true. Everything else can help or hurt, depending on the application and how doctrinaire the application of a given approach/methodology is, the organizational knock on effects (e.g. "Mr Tough Guy Testalot" holds up the release train or nukes your architecture to make it 'testable'), etc. but, seriously, "great developers who ship" is really what moves the needle.
Having a great Ops staff also helps ;) Of note is Thomas Limoncelli who wrote "The Practice of System and Network Administration" [1] and "Time Management for System Administrators" [2] works for Stack Exchange (formerly at Google). The Practice of System and Network Administration is basically the bible for most sysadmins, myself included.
ps. I only singled Thomas Limoncelli out as an example just to highlight the caliber of their Ops staff.
Every feature/user story has to go through a workflow of selected for development -> UX design(If required) -> Development -> Unit Tests(Or the otherway around) -> Staging -> Load Test -> Acceptance Test -> Production -> Analytics(To see if people actually use it) -> Learn from analytics -> back to start if required.
The goal is to get as many issues through the workflow as fast and rigoursly(no shortcuts) as possible at a sustainable pace. Have a continuous flow of features rolling out through this process. Ideally with continuous delivery to automate the majority of it.
Everything else is noise. If you have great developers who ship, then by definition you don't have doctrinaire methodology or "Mr Tough Guy Testalot" (I generally find "Mr No Test" to be a much bigger problem anyway). You might the situation where you have great devs but bad management, but that's next to impossible in the real world.
There's really only two steps to great software development.
For piths sake, I want to say "Everything else is noise" but that isn't true. Everything else can help or hurt, depending on the application and how doctrinaire the application of a given approach/methodology is, the organizational knock on effects (e.g. "Mr Tough Guy Testalot" holds up the release train or nukes your architecture to make it 'testable'), etc. but, seriously, "great developers who ship" is really what moves the needle.