Only if you want it to run VERY slowly, have a limited choice of hosting sites and if you especially want it to get hacked.
.net/ASP runs only on IIS. Only %20 of the web servers out there if that many use IIS. Gartner group at one time strongly recomended ditching IIS to all business customers who used it. So your hosting sites are limited. Apache runs on all major platforms. There is an open source project called mono to provide .net programming languages to other platforms. Unsure about compatability or how advanced the project is.
IIS has severe chronic security problems. IIS only runs under Windows which again has severe chronic security problems. IIS patching is iffy at best. When I admin'd IIS servers I had to keep an entire machine around purely for testing patches. At least 1 in 3 when first released would crater the server, sometimes bad enough I had to rebuild the whole thing. I spent more time on IIS severs which were less than a fifth of my web servers than the rest combined. Then you have to buy a windows OS to run it on since IIS only runs under windows.
PHP is THE most widly used web side scripting language. So many tools, templates, examples, classes and other programming advantages exist that you can almost write massive complex apps without touch more than a few lines of code. The support base is extensive. PHP is dramatically more secure than .net/ASP.
Any other language you are likely to talk directly to the DB. .net/ASP talks to MDAC dlls. which have a whole series of insecurity issues. It's also yet ANOTHER thing you have to take care of, deal with and debug sometimes. Then you have to patch it from time to time. It's another point of failure that does fail sometimes. It also can do wierd things to your SQL. At least it used too from time to time. I'd watch the SQL going in then the logs on the DB side and wonder how exactly that query got so badly mutilated inbetween. The middle ware MDAC software also slows down the transactions. One more process that has to put it's hands on the data.
In terms of pay. PHP jobs are more common, pay about the same as .net/asp. If you want to freelance PHP is the language for you, almost all the freelance jobs out there are PHP. Java coding and .JSP coding are the high pay jobs in web development. As for ease of use nothing beats Coldfusion though PHP with templates and classes makes creating complex apps easier than coding in Coldfusion. Coldfusion has the easiest syntax to learn, but Macromedia has been steadily killing it since they bought it from Allaire.
As for coding, with .net/ASP things built into most other languages have to be constructed. Most ASP coders build a set of functions for themselves to replicate what is already availible in other languages.
.net sits atop XML. Why not use AJAX and raw XML instead, going to be faster. You can use Java web services, the classes are actually quite good and minimize typical coding time. Tomcat a good server for it, but you have your choice of several if you use Java Web services. If your programming for the big boys then that's what you'll be using.
Browser compatability. Most sites today are showing Firefox as the leading browser. In any website you really have to support both Firefox and IE. .net/ASP makes it really easy to write IE specific code. That doubles your work as you have to go back and write a WC3 compliant branch plus an older IE branch for areas where you accidentally wrote propriatory IE specific code.
As for long term use. Microsoft technologies are intentionally destroyed every few years as a sales gimic and I think sometimes just out of pure sadism and hatred for the users of the technologies. First it DAO, then RDO, the direct connect then and so on until you just find a vendor that works and skip the Microsoft technologies completely. I used Quick C but they dropped it and I discovered Turbo C and never intentionally used a Microsoft C compilor again. QB and the proffesional version PDS was what Microsoft told everybody to use for a time. Then they dropped support without warning and Foxpro was thier new baby. Millions of lines of code later it was Visual C that everybody was supposed to use and support all but ended for Foxpro. Then it was VB. Once billions of lines of VB code were written and millions of companies dependent on it they rewrite the whole thing so your VB code is no worthless. They call it .asp and everybody is supposed to use that buggy monstrosity. When folks didn't, they created imitation Java with the .net studio. Again your hard learned skills are religated tot he trash. No real carry over. Learning Microsoft languages is a lesson in frustration. Don't worry about bugs, they'll ditch the product before they fix the bugs. It's designed so that you write the same app over and over and over and over again. Just in different languages. Usually with fewer features and more bugs. It's gotten where DOS users in the 80s with ugly command line screens had considerably more powerfull applications on thier desks than most users of custom software do today. Computer users are being dragged backwards by this constant rewriting of thier apps to run on new platforms and languages. There are still tens of thousands of loyal QB, Foxpro and other deserted Microsoft languages. All stranded with dead languages and no upgrade path or backwards compatability. It is costing the consumer money out of their pocket with the reduced efficiency and constant need to buy new compilors/packages. The expense passed on to the consumer of course.
In short not only is it a bad idea it is a REALLY bad idea to build anything in .net/ASP.