The frustrating thing is just the inconsistency of it all. Watching Brighton/Chelsea now. Ref gives a pen, called to look at the monitor and it clearly wasn't a pen, ref rescinds it. THAT'S how it should work.
Unfortunately there's too many twats out there who can't properly manage the system, plus there's still so much gray area when it comes to what's a pen, what's a handball, etc.
The only thing I'd keep is the offside technology.