MP really isn't ready for primetime use. It's still missing a lot. And the UI is... pretty bad. Needs a major overhaul.
As of 12.0, a handful of the things keeping me from switching over:
-No scene transitions (yeah it's minor, but jump cuts look crappy)
-Can't crop sources separately per-scene (so if you need a closer cropped cam just in your in-game scene, you're boned)
-Can't enable the encoder without going online, to test load
-No ability to lock the scene layout to avoid accidental element moves
-Onscreen positioning options (center horizontally, especially) are needed
-Can't move multiple elements at the same time
-Can't duplicate scenes (for working off a master template)
-Can't auto-record when streaming
-Cannot set a gamecap hotkey (the auto-selection is dodgy for some games)
-Can't lock-on Projector mode (hitting ESC exits it, which sucks if you want it on an audience projector)
There's more, but the list rolls on. Really though, the UI is one of the biggest stopping blocks. It was clearly designed in an attempt to be 'newbie friendly' while sacrificing usability and hotkeys for the power-user end. It's badly cluttered, cryptic in many ways, and is an outright roadblock to any kind of smooth production workflow. It's really, REALLY bad.
Plugins will come as the program matures further, probably after feature-parity. At this point, most I know on the Windows side are sticking with Classic. I try MP again every few versions, and am reminded each time that it's still in early development, and will get better with more time in the can.