Just for additional information, here is the usage notes from a dictionary :
When what is the subject of a clause, it may be construed as singular or plural, depending on the sense. It is singular when taken as the equivalent of that which or the thing which, as in I see what seems to be a dead tree; and it is plural when it is taken as the equivalent of those which or the things which, as in He sometimes makes what seem to be gestures of aloofness. - When a what clause is itself the subject of a sentence, it may be construed as singular or plural, but the conditions governing this choice are somewhat more complicated. In general, a what clause will be taken as a plural when the clause contains an explicit indication of its own plurality. There are two principal cases. First, the clause is plural if what is the subject of the clause and the verb of the clause is itself plural: What seem to be two dead trees are blocking the road. What most surprise me are the inflammatory remarks at the end of his article. If the verb in the what clause does not anticipate the plural sense of the predicate in this way, a singular verb is generally used in the main clause as well, though the plural is sometimes found: What truly commands respect is (sometimes are) a large navy and a resolute foreign policy. Second, the what clause is treated as plural when its predicate contains a plural noun phrase that unambiguously establishes the plurality of the clause as a whole, as in What traditional grammarians called "predicates" are called "verb phrases" by modern linguists. What the Romans established as military outposts were later to become important trading centers. In the absence of explicit plural marking of either of these types in a subject what clause, the clause is usually treated as singular for the purposes of agreement, regardless of the sense: What she held in her lap was four kittens. What the apparent diamonds turned out to be was paste. In some cases, however, a clause with what as the subject may be treated as singular or plural, depending on a subtle distinction of sense. In What excite him most are money and power, the implication is that money and power are distinct elements; in What excites him most is money and power, the implication is that money and power are taken as constituting a single entity.