He's not intricate enough to play in the final third; he hesitates, does that daft shuffle and makes the wrong pass or lays it back 5 seconds too late. Dele Alli in the same area is much more decisive.
The only way Barkley becomes a top player is if he has the humility to accept he's not a #10 and focuses on becoming a more orthodox midfielder. The best game I've seen him have was against QPR where he tackled everything, passed simply and quickly and drove forward from deep and scored a worldie, all from the '#8' role.
His short possession passing is very good, he's got very clean feet and has the physicality to boss the midfield. As I said earlier, he requires ground infront of him to instinctively burst into without thinking, and the congestion of a Premier League central midfield would force that out of him. He's also got a cracking shot on him, but generally [Poor language removed] about too much rather than unleash one from range.
Again, a lot is down to Martinez telling him he's a final third player when a proper coach like Mourinho would have come down hard on him and made him understand where his strengths really lie.