One big difference? That would be demographics. Reagan carried about 58-59% of whites in 1980 and won a landslide as a result. Bush the Elder did about the same in 1988 and won a similar landslide. W did about the same in 2004 and won an uncomfortably close reelection. Romney carried about the same and lost easily to Obama.
I hope Trump wins. I think he has a decent chance, but demographics make it much harder for him than it was for Reagan.
There is a lot of anecdotal evidence that Trump is doing unusually well with minority voters.