A common objection to Mono is its fuzzy legal position. Although parts of .NET are ECMA standards open for anyone to implement, other parts (including ASP.NET) are not. Mono implements ASP.NET as well as other non-ECMA class libraries. The worry has been: if Mono gets successful enough to threaten Microsoft, will there be legal problems?
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See the comments below for more. It seems this falls short of a promise not to sue over patent breaches in Mono, if they exist, except in certain defined circumstances (Novell customers or non-commercial). I guess that is not surprising.