What version of .NET Framework does Windows 10 contain by default?
According to MSDN
versions 4.5 and later should be determined by Release value of registry subkey
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full
In preview version of Windows 10 which is available today this value equals 393295. In MSDN we find that the latest value:
393273 - .NET Framework 4.6 RC
So probably the final build of Windows 10 will contain .NET Framework 4.6 but actual value of Release can change, will be equal or greater than 393295.
.NET Framework 3.5 is not released by default with Windows 10 (as well as with Windows 8) but can be installed optionally without any problem.