My personal experience is that the MS stack is tough for a startup because Sql Server can get expensive... especially if you want uptime. The feature to rebuild the indexes (without downtime) and do horizontal partitioning requires the enterprise edition... and it retails at 25k/processor.
If you go with asp.net consider your budget before choosing sql server.
Most places can take the few minutes of SQL blocking resulting from an index rebuild in standard edition.
I know we often jump towards 5 9 solutions, because it feels like the "Right Thing(TM)" to do, but you can afford a lot of episodes of few minutes of blocking for what enterprise edition will cost... (we do run enterprise edition on our needed-by-cash-register DBs)
I'm a big fan of MS-SQL, and would encourage people to think about the value of chasing that 4th nine of uptime. It doesn't make sense for most companies, IMO.
Nothing stopping you using another database server - I've started using CouchDB with an ASP.Net MVC application because I wanted documented oriented storage and it works perfectly well.
MySQL seems to have plenty support, as does Oracle etc.
If you go with asp.net consider your budget before choosing sql server.