I created a simple test add-in in VS 2010 and moved the solution to VS 2012, targeting .Net 4.5.
The first issue I have is that VS 2012 can't find the ArcGISAddInHostGenerator, so editing the .esriaddinx doesn't reflect on the designer.cs.
An other one concerned the <AddIn language="CLR4.0"... /> attribute. If left to "CLR4.5" the add-in is recognized by the arcmap add-ins manager, but the main toolbar doesn't show. If left to just "CLR", the toolbar is visible but the commands display a "Missing" caption. If left to "CLR4.0", the add-in actually runs fine.
Then I tried to move an existing arcgis 10 add-in to arcgis 10.1, VS 2012 and .net 4.5, but I can't get it to work. If I leave the language attribute to "CLR4.0", the add-in compiles but arcmap crashes upon startup.
Can anyone that has gone through this share some light?
Thanks
Edit: I'd forgotten to uncomment <supportedRuntime version="v4.0.30319"/> in arcmap.exe.config. Now I fixed some errors and the add-in works. Only problem that remains is the missing ArcGISAddInHostGenerator