According to Microsoft:
The Microsoft .NET Framework 4.5.1 is a highly compatible, in-place update to the Microsoft .NET Framework 4 and the Microsoft .NET Framework 4.5. The offline package can be used in situations where the web installer cannot be used due to lack of internet connectivity.
Some known issues with .NET Framework 4.5.1 are given in KB2890857.
New features of .NET Framework are:
- Performance and reliability improvements
- The ability to suspend and resume operations in the ASP.NET page framework
- The ability to compact large object heaps on-demand
- 64-bit Edit and Continue controls
- The ability to trace and sample activities
- Improvements that make SQL connections more resilient
- The ability to manage return values