1. Version control
2. Issue tracking
3. One-click deploy
4. Internationalization
5. Security
before you start building your application, then you build and release it, then (and only then) start thinking about:
6. Email
7. Web services
8. Scalability
9. Statistics & monitoring
10. APIs
This mostly squares with my experiences (both with my own startup - currently on step 3, with 4, 5 and a launch-ready app already done - and working for others). The only changes I'd make are:
1. Move statistics and monitoring up the priority list, before launch. You want that data available to drive feature implementation.
2. I'm of two minds on internationalization. I think that most apps can wait until they're popular before they need to internationalize. However, i18n is really difficult to do later, after you've already built an app. I watched LiveJournal go through the process, and it wasn't pretty. So even though you don't need it, you may want to do it upfront because it'll be much harder later.
[0] Carson Workshops, 'The Future of Web Apps, carsonworkshops.com/summit, Cal Henderson'
http://media.libsyn.com/media/carsonsystems/Cal_Henderson.mp3
[1] Carson Workshops, 'Building Enterprise Web Apps on a Budget How We Built Flickr, ~ 10Mb, 10 min.'
http://odeo.com/audio/1207043/view
[2] O'Reilly, 'O'Reilly, Building Scalable Web Sites, Cal Henderson. Chapter 5: Data Integrity and Security, ~ 5Mb'.
I'm going to have to check out Cal Hendersons book sometime but first I have to make a good site then I can worry about scaling it.