- He's rich, so he can't be bought. (Or can he? See below.)
- He doesn't care about political correctness.
- He's a political outsider.
- He says things other candidates are afraid to say.
- He doesn't back down.
- He refuses to play by the establishment's rules.
- He believes the Iraq War was a bad idea, so he's better than average on foreign policy.
- It's questionable whether or not he really believes most of what he says (typical of political figures). He might be saying what he thinks conservatives want to hear.
- He's against free trade.
- His plan for immigration reform (seen by many as his biggest strength) is unrealistic and economically short-sighted.
- Despite his personal wealth, he may be beholden to even wealthier backers. I don't know this for a fact, of course, but it's a possibility. Even the wealthiest candidates rarely finance their own campaigns.
- He's an authoritarian. He may very well attempt to push the limits of executive power if elected (like Obama . . . and Bush).
- Even if he's right on a few important issues, it's likely due to pragmatic concerns rather than principle.
Trump's drawbacks are shared by most of the other candidates, while his strengths are mostly unique. His immigration plan is terrible, but it probably won't be implemented even if he wins. Trump would probably not make a good president. Despite all this, I still think he's better than most of his competitors (Carson, Bush, Fiorina, etc.).