It didn't feel empty to Nadal. It meant he had now completed the career Grand Slam, something only Laver, Agassi and Federer had ever managed to achieve in the Open Era!
Mind you, I think his 2010 USO win was a little hollow compared to Del Potro's 2009 win and even Murray's 2012 win:
In 2009, Del Potro had to beat both him and Federer, the defending champion, back to back.
In 2012, Murray had to beat Djokovic 2.0 or at least 1.5, the defending champion who was a 5 times Slam winner and had already won the AO earlier that year.
In 2010, Nadal only had to beat Djokovic 1.0 whose last Slam final had been 2 and a half years earlier at 2008 AO where he beat unseeded Tsonga. At that stage, Djokovic was still Nadal's pigeon.