Hi,
License is a problem, noncompacting GC is another problem, there is no WPF, WF, Entity Framework, has only basic WCF, MonoDevelop has a long way to race with Eclipse/VS.Net, mod_mono isn't stable or perfect as tomcat/IIS, I couldn't find any high traffic site using it...but, You probably will not face with a serious problem, if your project isn't enterprise level. C# is probably the best architected language ever and evolving fast. .Net framework is far better than jdk in my opinion and mono people working hard, but there are other problems as I said; another choice is Java. Java has the mature / rack-solid state of art projects like Terracotta, Hibernate (ported to .net), Ehcache, Compass,... but some people say that Sun is out of money and Java isn't evolving for years (as a language) so began to die,...
I think the most important thing is finishing the job done right and on time. Select the technology which you know best and like most and don't waste your time by looking back. It'll be not a problem until your project grows too much.
Cheers,