That's a really good suggestion. The live demo was a bit of an after thought on my part. I bought the design from Theme Forest, customized it, and launched it in a few days, then realized it would be good to see the tool in action somewhere.
We're kind of pivoting towards a more dashboard-style product for professional, highly-paid writers (lawyers and speech writers mostly), and once I'm done with that we'll certainly have a cool live demo ready. We'll also probably release a scoring API with free and for-pay rate limits, since developers have already shown some really creative applications [1, 2] in our hackathons.
[1] http://blog.effectcheck.com/2011/07/14/interview-with-joey-c...
The add-on program seems pretty cool too. We're planning on adding EffectCheck to the list of supported APIs soon.
I would, however, encourage you to use .NET MVC instead of WebForms. The former is very similar to Rails, Django, and other popular MVC frameworks, so if you find that you'd like to make a transition to another language/framework, it will be much easier this way. The latter is really just an antiquated technology with lots of problems and not very much of an OSS community.
At the end of the day, it's about getting to market fast and testing your hypothesis. Building your MVP is just the very beginning of your startup journey. You will probably have to change your product a lot, so go with what you can hack in quickly. If it turns out that you have the next StackExchange (which is built on .NET MVC) and you need to massively scale, then drop Spolsky or Atwood a line and I'm sure they'll be happy to give you some pointers. That's a good problem to have.
[1] http://effectcheck.com
[2] http://www.nashcoding.com/2011/04/10/building-a-startup-part...