Grandy is heavily overrated by both the Tigers and the fan base. The thing with Curtis is, he's the most professional, well-behaved, likeable baseball player imaginable. He's the kind of guy you'd want your daughter to date. He hustles all the time, catches the ball with both hands and runs out every ground ball, no matter how futile. He's clean-cut, well-groomed, articulate, modest, respects his elders and the coaching staff, gives time and money to local charities; the list goes on.
As a ballplayer, he has nice range in CF, but he misplays the occasional ball and sometimes turns a routine fly-out into an adventure. He has a so-so arm. He can draw walks and he strikes out a lot. He has good speed and power. Combine that with the ballpark he plays in and you understand how he had the 20/20/20/20 season a couple years ago.
The problem is, Curtis Granderson cannot hit a curve ball. At all. I've watched him play for 4 years now, and I can honestly say I don't ever recall him really making solid contact with a breaking pitch. He obviously has recorded a few base hits, but it's a miracle if he really connects with one.
The only reason Granderson is still in the major leagues is most pitchers still throw him an occasional fastball, usually on the first pitch. He hits a lot of those out of the park, at least against righties. I guess they reason that even when a batter has a big hole like that, you still have to mix it up once in a while. I disagree; I don't think he could hit breaking balls even if he received a 100% diet of them. I wouldn't even mix in an occasional fastball until he proves he can consistently hit a curve even when he's sitting on one.
The best part is that Jim Leyland has seemingly permanently etched Granderson into the leadoff spot, despite the terrible fit. Nothing like facing a LHP and starting the first inning off with an automatic out.
I'm not sure who they'd replace him with, though.
You're too hard on Granderson. He'll never be the player he was in 2007; what looked like a breakout year at the time now looks like a career year. Even so, Granderson is a league-average run producer who plays league-average defense in CF. He more than earns his paycheck.
Your eyes are playing tricks on you with the curveball. Check out his Pitch Type Values on his Fangraphs
page. The only pitch Granderson has trouble handling is the slider. Naturally, he sees a lot of them-- but that was true in 2007 also, and he handles the slider better now than he does then. He's always been slightly above average against the curve ball. The biggest difference between the 2007 and 2009 versions of Granderson is that the 2007 version murdered fastballs; the 2009 version was only league average against the fastball.
His OBP did take a tumble-- from .361 in 2007 and .365 in 2008 to just .327 in 2009. During that time, his BABIP slid from a ridiculous (and unsustainable) .362 in 2007, to a solid .317 in 2008, to a hideous .276 in 2009. (The league average is just over .300.) His walk rate did dip slightly, as did his ISO; meanwhile, his K rate increased slightly. He also hit dramatically more popups in 2009 than he did in prior seasons.
There are two theories that could explain this constellation of data:
(1) in 2009, at age 28, Granderson began his decline phase; or
(2) Granderson suffered through the worst slump of his career late in the season and had generally bad luck on balls in play, but he's nearly as good a hitter entering 2010 as he was in 2008 (accepting that 2007 was a career year).
You watched him play, so I'll defer to your judgment. Sitting in front of my computer terminal, however, (2) looks more plausible to me than (1).