ASP.Net controls, postbacks for everything, view state, a general hiding of the actual power of ajax and an obsession with using tables made .Net a terrible web development environment if you had any clue what you were doing. The html it spat out was a bloated horrible mess, web services were anything but unless you really slogged your way through some pain, ashx pages and the directory structure just felt plain wrong.
That said I'd almost, almost, switched to using python/django and then I had a look at MVC and it did everything I needed. Full control over html, nice code separation, a nifty ORM for taking away the pain of basic crud. An alright templating system easily extensible using extension methods. And 3.5 brought some tricks using LINQ and anonymous methods.
I still know I need to learn a few more languages, but MVC has allowed me to copy everything I felt was the 'right' way of developing webs apps from rails/django without having to actually learn a new language and dev environment.
If you weren't worried about performance and didn't want to learn how to actually program, the old ASP.Net was ok for very quickly throwing out web apps for the 10 people in HR. Much more than that required an in depth of knowledge way beyond most .Net programmers, with most of those people being multi-lingual anyway and tending to opt for the cleaner code the alternatives offered.
I don't feel that's the case anymore, but it's way behind in adoption at the moment.
You still hit the occasional WTF in MVC where if feels as if someone wrote a part of the system without reading the memo about flexibility (the validation solution in MVC, for example, is terrible, although not finished and LINQ to entities can be frustrating).
tl;dr;? No startups because until recently good web development in .Net was actually really hard. MVC's changed that.