L4 is based on .MONO which many of us on the Mac felt was a mistake and the source of bugs and slowness (although at first our concerns were dismissed).
I haven't seen any information about what the underlying engine is that drives L5. From some comments it appears that the .MONO philosophy has been abandoned. I sure hope so.
Is L5 now written in Java? Some other multi-platfomr foundation? This information could relieve my concerns and influence how soon I upgrade.