Compatibility

The easiest way to describe what Mono currently supports is:
Everything in .NET 4.5 except WPF, WWF, and with limited WCF and limited ASP.NET 4.5 async stack.

System.Web and WCF are candidates for ‘almost immediate’ porting from the .NET reference source back to Mono, so support coverage may improve.

Here is a slightly more detailed view, by .NET framework version:

Implemented

Partially Implemented

Not Implemented

.NET 4.6

C# 6.0

.NET 4.5

	C# 5.0 - async support
	Async Base Class Library Upgrade
	MVC4 - Partial, no async features supported.
	MVC5 - Partial, no async features supported.
	ASP.NET 4.5 Async Pipeline - Needs a parallel processing pipeline with async support, not done.

.NET 4.0

	C# 4.0
	ASP.Net 4.0
	ASP.Net MVC 1, MVC 2 and MVC3
	System.Numerics
	Managed Extensibily Framework - Shared with .NET via MS-PL license
	Dynamic Language Runtime - Shared with .NET via MS-PL license
	Client side OData - Shared with .NET via MS-PL license
	EntityFramework - Available since Mono 2.11.3.
	Parallel Framework and PLINQ
	CodeContracts - API complete, partial tooling
	Server-side OData - Depends on Entity Framework.

.NET 3.5

	C# 3.0
	System.Core
	LINQ
	ASP.Net 3.5
	ASP.Net MVC
	LINQ to SQL - Mostly done, but a few features missing

.NET 3.0

	WCF - Silverlight 2.0 subset completed
	WPF - No plans to implement
	WWF - Will implement WWF 4 instead on future versions of Mono.

.NET 2.0

	C# 2.0 (generics)
	Core Libraries 2.0: mscorlib, System, System.Xml
	ASP.Net 2.0 - Except WebParts
	ADO.Net 2.0
	Winforms/System.Drawing 2.0 - Does not support right-to-left

.NET 1.1

	C# 1.0
	Core Libraries 1.1: mscorlib, System, System.Xml
	ASP.Net 1.1
	ADO.Net 1.1
	Winforms/System.Drawing 1.1
	System.Transactions
	System.Management - Does not map to Linux
	System.EnterpriseServices - Deprecated