What has he shown so far?
So far in his career he has had 1 good season in which he hasn't exactly set the world a light but to be fair to Shaw he's been ever present this year for Southampton being their first choice left back thorough out the whole season and both defensively and offensively doing a good job.
So in that case he has shown that he can be a competent premier league player providing width and being decent defensively but is he really special? in the same way Ashley Cole was. Looking back at the world cup and seeing as Shaw was chosen but couldn't get in the team if he were that good surely he would have started so this makes us think if he isn't good enough to play international football is he good enough to be first choice left back for one of the biggest clubs in the world?
I think he has a lot to prove for Manchester united over the coming years and he really has a tough season ahead for him as Manchester united fans wont accept the excuse of "he's too young" or "he needs time " as this is a huge club that finished 7th last season and needs to get back in that top four without any excuses.
Age on his side
First and foremost the thing that stands out as an obvious positive is his age which means he has time to get better and improve but this will mean he cant be first choice left back and will have to be used wisely in certain games but that will mean he can learn off a more experienced left back.
If im being totally honest what iv saw so far from Shaw doesn't warrant 30 million however considering the amount of games he has played at such a young age at club level makes me think if he improves his reading of the game and then his creative skills going forward I don't see why he couldn't become a top fullback. Shaw is a good professional he looks as if he will give you 6 or 7 out of 10 each week no lower no higher but my my instincts are saying man united may need a little bit more from him lets face it he's not unbelievable defensively like Ashley Cole was at his age , doesn't have a world class left foot like Baines and isn't the great dribbler Bale was.