Compatibility

The easiest way to describe what Mono currently supports is:
Everything in .NET 4.7 except WPF, WWF, and with limited WCF and limited ASP.NET 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 4.7

C# 7.0
ValueTuples

.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