Trump shares one problem with Jeb Bush and Hillary Clinton. No undecideds.
Either you love Trump or you hate him. There are very few undecideds in the squishy middle that he can pull over to his campaign. There are no voters that have not heard of him or already formed an opinion about him.
When a candidate is unknown (like Obama in 2008), it is much easier to convince the squishy middle undecideds to swing your way. People have a tendency to assume that the unknown candidate is much better than he is. That is why in most polls pitting an unnamed opponent against an incumbent, the unnamed opponent usually comes out on top.
Many if not most of the undecided squishy middle voters are too stupid to be allowed to vote. Unfortunately, they are allowed to vote (and frequently delivered to the polls by Democrat activists) and there are enough of them that they usually determine elections.
If, say, 5% of Republican voters decide to stay home because Trump is the nominee, we end up the same place that Romney left us -- because 5% of the Republican voters stayed home because either a.) Romney wasn't conservative enough or b.) he was a Mormon.
In 2016, the 5% could range from a.) conservatives who recognize that Trump isn't really a conservative to b.) RINOs who are appalled at Trump's often unconventional behavior.
Consequently, I should think that Trump becomes a problematic nominee -- he risks losing the election by losing a potentially decisive segment of the Republican vote.
Supporters of his nomination must, therefore, be prepared to take this risk -- while hoping that Trump will draw enough support from previous non-voters and some Democrats to compensate.
Maybe he will...maybe he won't.
It's certainly true that almost everybody has already heard of Donald Trump. And it's also likely that he's polling closer to the top of his range than other candidates (because he's polling better than other candidates).
But is it really the case that people either love him or hate him? Trump's shown enough ability to surprise people up till now that I can see him going any number of ways from here and my attitude toward him changing and developing to reflect what he chooses to do.
When a candidate is unknown (like Obama in 2008), it is much easier to convince the squishy middle undecideds to swing your way. People have a tendency to assume that the unknown candidate is much better than he is.
Sure, people of very different points of view assumed that Obama agreed with them the first time he ran, because they didn't know much about him. Even though we know more about Trump now than many people did about Obama then, all kinds of hopes build up around a candidate from outside the usual political world -- somebody who hasn't had a chance to fail at politics or alienate voters yet. I think Trump is better off than Jeb or Hillary in that respect.