No one thing. Just as there are different sorts of poet and different sorts of scientist, there are different sorts of philosopher. What made Kierkegaard a great philosopher was not the same thing that made Carnap a great philosopher. If one has to generalize, I would agree with Myles Burnyeat who once said that philosophy needs vision and arguments. Burnyeat's point was that there is something disappointing about a philosophical work that contains arguments, however good, which are not inspired by some genuine vision, and something disappointing about a philosophical work that contains a vision, however inspiring, which is unsupported by arguments. (Andrew Pyle (ed), Key Philosophers in Conversation, p.44)
He did not elaborate on these remarks, simply adding that "'vision', 'argument', and 'support' can mean many different things". These words certainly can mean many different things, but whatever they mean, it seems that the two kinds of disappointment Putnam spoke of can still be characterized a bit further in a completely general way: Arguments not inspired by some philosophical vision are disappointing because it is not even clear why we should take them seriously in the first place; a philosophical vision not supported by arguments is disappointing because it may very well be nothing but an intellectual fantasy. To mimic Kant: Arguments without a vision are blind; a vision without arguments is empty.
The relation between philosophical visions and arguments is, to some extent, analogous to that between scientific theories and confirmatory experiments. There is, however, an important difference. A scientific theory usually carries implications for how it can be confirmed experimentally, but a philosophical vision does not carry implications for how it can be supported by arguments. If there is a theory (about some kind of natural phenomenon) such that no experiment can be designed, at least in principle, to confirm it, it is doubtful whether it should be considered scientific at all. By contrast, a philosophical vision that is not supported by arguments may not be considered any less philosophical. Another way of putting it is that philosophy is more tolerant of empty talk than science is. This may be, alas, why there is more bad philosophy than bad science.