While Howard leads all of MLB in HR and RBI by a wide margin, he's not having quite as good a year as you might think. (I know that sounds crazy.)
First, check out this list of lowest OPS+ values for a guy with 47+ HR. It's been done 92 times, and only twice has a guy been under 130. Think about his OPS+ of 121...does that seem like a number that a guy leading all of baseball in HR and RBI should have?
A large part of it is his low batting average. Two years ago, he hit .313 and slugged .659 with an OPS+ of 167. This year, he's batting only .248 and slugging .537. That's 65 points off his batting average and 122 points off his slugging average. His extra bases are roughly the same--the difference is actually about 30 fewer singles, and the fact that he's got more ABs this year.
His OBP is also way down from .425 to .337, thanks partly due to the low BA again, but also due to the fact that he's walking less. He already has more PAs in 2008 than he did in 2006, but he has 27 fewer walks at the moment.
I'm not suggesting that Howard is a dud. However, it's also true that his 2008 is not in the same league as his 2006 was, or even his 2007.