What qaz said. Essentially, what he's done is say all the crazy stuff that past candidates said that was popular with a certain sect of voters in order to win the nomination by riding that sect of voters.This is not something done in haste. This disgust with the Establishment (particularly the Republican Establishment) has been building for many years (even before Obama got elected to his first term and going back to the GWB days). What has happened is that Trump has given a strong voice to the concerns many Republicans have had and which the Republican Establishment has nearly completely ignored. The Democratic Establishment has basically refused to acknowledge it at all.
He can't actually DO any of it, but who cares! Trump Trump Trump!
It's the exact same thing that Bernie is doing. Neither one of them can actually execute half of what they think they can.
So "what's the big deal about the wall?" It's a total waste of money. It doesn't solve anything (because yes, we ARE worried about our northern border security as well... there's evidence of its use by some not-so-good people, but we kinda ignore it because Canadians aren't perceived as a threat to the country even though they're here taking jobs and speaking foreign languages too!), and all it does is alienate the lower half of this hemisphere in a time when we should be trying to work with those folks who don't look like you and me.