Pablo Sandoval is a good baseball player, but he is not worth $100 million.
As the nickname implies, Kung Fu Panda has no speed and is never going to win a Gold Glove at third base. All of his value comes from his bat, which is good, but hardly $100 million good.
First off, Sandoval's power is merely adequate for a corner infielder. Though he's shown the ability to hit for power before, exceeding 20 home runs and slugging over .550 in 2009 and 2011 (not to mention that Reggie Jackson-esque three-homer game in the World Series two years ago), his .176 career ISO is indicative of a player with merely average pop. Look no further than his middling home run totals; Fat Ichiro has failed to top 15 home runs in three of his past four seasons and his career high is 25. He can't be counted on for 20 home runs in a season, much less 30.
Consequently, Sandoval doesn't drive in nor score many runs. He's never scored 80 runs in a season and has driven in that many only once--in his career year of 2009. Obviously runs and RBI are team-dependent stats, but the point I'm trying to make is that because Sandoval doesn't hit many home runs, he doesn't drive himself in as often as guys like Miguel Cabrera and Chris Davis. His poor runs and RBI totals reflect that.
Another explanation for Sandoval's lack of runs (besides his non-existent speed) is that he doesn't get on base a whole lot. His .347 career OBP is good but hardly great, and the reason is that he doesn't walk very much. His solid on-base skills stem mostly from his ability to hit for good averages, as he's a .293 career hitter. But his averages have declined precipitously over the past three years, falling from .315 in 2011 to .283 in 2012 and .278 last year.
With strikeouts and defensive shifts on the rise, hitting for average is becoming increasingly difficult nowadays, and it's only going to get harder for Sandoval, who can't leg out hits the way Mike Trout and Billy Hamilton can. If Sandoval's declining batting average trend continues, or even holds steady at the .270-.280 range, he's going to lose a lot of offensive value unless he starts taking more walks (which, given his propensity for hacking at pitches off the plate, seems unlikely).
Durability's been an issue for Sandoval as well. The hefty third-sacker has averaged 134 games in his five full seasons, a solid number, but by missing 28 games per season for five years he's lost 140 games, almost the equivalent of a full season. Over the last three years his attendance has been even poorer, as he's averaged just 122 games per season. Typically players miss more games as they get older, which doesn't bode well for Sandoval's health going forward.
Now is also an appropriate time to mention that his Prince Fielder-ish body probably isn't going to age well. Weight has always been an issue for him so unless he takes a cue from CC Sabathia and David Ortiz and slims down considerably, his decline is likely going to hit like a ton of bricks. Chunky players like Sandoval tend to lose it overnight, as opposed to the more athletic types who age slowly and gracefully.
The best thing you can say about Sandoval is that his floor is pretty high. The two-time All-Star has never been terrible, or even below average. In his worst year, 2010, he still popped 34 doubles, put up a .732 OPS/99 OPS+, and was worth about a win and a half above replacement. His ceiling is also pretty high, as he was borderline MVP-caliber in 2009 and 2011.
One would think a potential away from San Francisco's pitching-friendly confines would help his numbers return to their '09/'11 levels, but in reality he's been a much better hitter there than on the road. At AT&T he's a .313/.366/.494 hitter: everywhere else he's a .275/.330/.447 hitter, essentially the difference between Sandoval at his best and Sandoval at his worst. His splits suggest that he's a much better left-handed hitter, though, so maybe he'd thrive in a park well-suited for southpaw sluggers. I hear the Yankees might be in need of a third baseman soon...(don't tell Alex Rodriguez that, though)
The bottom line: Sandoval is a good hitter, but that's all he is, and he probably won't be very good for much longer. The 27 year-old is off to a terrible start this year, and only time will tell if it's nothing more than an early season slump or the beginning of a premature decline.