As of Nuget 3.4/Visual Studio 2015, NuGet now uses a much stricter policy for semantic versioning. c.f. http://docs.nuget.org/Create/Versioning#Normalized_Version_Numbers
In order for Nexus to behave correctly, it will need to address how it sorts version numbers, and how it decides packages are pre-release or release.
Unfortunately, there are some popular package on nuget.org that don't abide by this strictness - NLog, for instance, has gotten into the habit of labelling (in ODATA) versions like 4.4.0-beta3 as release versions.
As of NuGet 3.4, the (new) Normalized Version ODATA field is the definitive version for uniqueness purposes, as well as the definitive indicator as to whether a package is pre-release or not. Visual Studio will actually prune packages with pre-release semvers out of the search results it gets from Nexus.
Unfortunately, for now, Nexus defers to the metadata, not the semver. As a result:
1. Create a Nexus proxy to nuget.org
2. Open Visual Studio 2015
3. Make the Nexus proxy the only nuget source
4. In VS/NuGet, search for 'nlog'. Currently, NLog 4.2.3 is the latest release version, according to semver. It should appear in the search results.
5. Now search for 'nlog', with the 'pre-release versions' option checked. Currently, this will bring down the badly-tagged NLog 4.4.0-beta3. Nexus now considers this the latest
6. Perform the first search again, looking for release-only versions of 'nlog'
=> NLog has now completely disappeared from the VS search results. Nexus is returning 4.4.0-beta3 as the latest release version (as per the metadata), but VS prunes it out (as per its pre-release style semver)