For what it's worth, ASP.NET MVC is much, much less heavy than the original ASP.NET (if that's what you're referring to). This is especially the case if you use the Razor syntax for your views.
This is something that always bothered me about Microsoft-related stuff. Right now I'm using Python and Django for web development. I'm also using Ruby on Rails for a side project. Having access to the source code is vital for me as I've become accustomed to reading a lot of source code. And reading source-code for lack of better documentation is sweet, but then I went further and for instance I also copy/pasted a lot of snippets straight from Django's source code, or worked-around bugs by patching components.
That's why I consider open-source to be superior, regardless of all the polish that Microsoft is able to apply to their products. I'm a software developer, not your average user. Just as a sports-car racer would find unacceptable the lack of access to the internals of her own car, I find unacceptable the lack of source-code that I can read, modify and distribute.
For pure debugging purposes, have you tried using a decompiler? For reasonably well-written (and not obfuscated, naturally) code, the decompiler output is remarkably clear.
And thats if you even use that... I use 2 open source libraries to build web apps - Nancy & Simple.Data - and they are both f'in amazing. This is about as "light" as you can be. I have a prezi presentation on my blog about using these libraries to build web apps for a few talks I gave at code conf's. (thinkdevcode.com)