The first thing I noticed is that most of the browser games of that kind are written in PHP, which I knew very little and didn't like much. Every time I had an encounter with the PHP code, it looked to me way too messy, comparing to C# code. Maybe I just happened to see the work of messy PHP-programmers all the time, but that was my impression and even though I could read most of the PHP-code, I couldn't write it and starting a new language was not exactly in my plans.
Another argument I had was about databases – if I go with C#, then it will be MS SQL Server and if I go with PHP, then it will be MySQL. Two databases are very similar and use almost identical script language, but MS SQL Server is licenced (you have to pay for it) and MySQL is free. It is of course the matter of habit and preference, and in my case, the preference was clearly with MS SQL Server, which pushed the possibility of using PHP even further away.
So, after some consideration and finding more excuses for not starting with a new language that I didn't like too much, I decided to write a browser strategy multiplayer game in ASP.NET using my favourite C#.