In his "Computational Complexity" book, Papadimitriou writes:
RP is in some sense a new and unusual kind of complexity class. Not any polynomially bounded nondeterministic Turing machine can be the basis of defining a language in RP. For a machine N to define a language in RP, it must have the remarkable property that on all inputs it either rejects unanimously, or it accepts by majority. Most nondeterministic machines behave in other ways for at least some inputs ... There is no easy way to tell whether a machine always halts with a certified output. We informally call such classes semantic classes, as opposed to the syntactic classes such as P and NP, where we can tell immediately by a superficial check whether an appropriately standardized machine indeed defines a language in the class.
Several pages later, he points that:
language L is in the class PP if there is a nondeterministic polynomially bounded Turing machine N such that, for all inputs x, $x \in L$ iff more than half of the computations of N on input x end up accepting. We say that N decides L by majority.
Question 1: Why Papadimitriou concludes that PP is a syntactic class, while its definition is only slightly different from that of RP?
Question 2: Whether being "semantic" for a complexity class is equivalent to NOT having complete problems, or the lack of complete problems is thought of as a property that we GUESS semantic classes possess?
Edit: See related topic Do all complexity classes have a leaf language characterization?