Two Santorum flaws that immediately come to mind:
1) Pro-"manufacturing" industrial policy. One either understand and trusts the free market to allocate resources or else one does not. Santorum does not.
2) Sloppy and undisciplined messaging. Santorum was quick to say things like "UC campuses don't teach American history." or "Calls for increased college enrollment are elitist,", etc. There was some truth behind both of these lines but he let himself be mischaracterized and put on the defensive by going off half-cocked.
If there’s truth to it, then why are you criticizing him for addressing conservative issues that no one else would address. I teach history and I was thrilled to hear him calling out U of C for their terrible history courses.
I don’t see opposition to free trade as ‘deeply flawed’. I agree with you that it is a flaw, but I don’t think it’s significant enough compared to Romney’s and Gingrich’s flaws.