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,