The Carlson Lemmas in combinatorics. He told me at the time that he had struggled with the simple and natural way the problem was originally posed, but was able to push through a far more elaborate, stronger, and much less intuitive version.
Timothy J. Carlson
A dual form of Ramsey's theorem, (with S. Simpson) Advances in Mathematics 53 (1984), pp. 265-290.
Some unifying principles in Ramsey theory, Discrete Mathematics Volume 68 , Issue 2-3, (February 1988), Pages: 117 - 169