Unlike some other teams in this division, the Tigers are spending rather wildly in order to maximize their current window of contention. Justin Verlander is not what he once was, David Price may be leaving in free agency next year, and Miguel Cabrera is no longer otherworldly. He's just amazing. Just. At any rate, the Tigers basically have an aging, expensive core roster that, although still competitive, is seeing its performance decline a little bit each year. There are some younger bright spots, though.
2014 Season in Review
Those dang Tigers just barely beat out the Royals for the division title by finishing with 90 wins. Ian Kinsler turned in another solid all-around season with 5.1 fWAR generated from slightly above average offense, excellent baserunning, and great defense. Miguel Cabrera also had a 5.1 fWAR season mostly through offense. The two Martinezes J.D. and Victor had stellar offensive seasons. Overall, the Tigers had the AL's best offense by wRC+, but that offense was almost entirely concentrated in the forms of Cabrera and the two Martinezes.
On the pitching side, the rotation was still good. The pitching staff ranked 9th overall by fWAR. In terms of actual talent among the 5 starters with whom the Tigers ended 2014, Justin Verlander might have been the least talented (by ERA, FIP, and xFIP). Verlander's being paid the most, but the Tigers don't care much about that in the short term.
The bullpen was still bad. I am surprised to learn that Joba Chamberlain was their highest rated reliever by fWAR. Do you really need more information about the bullpen than that?
The Tigers had just enough to take the division, but they flopped against the Orioles in the playoffs. Despite throwing out Max Scherzer, Justin Verlander, and David Price as their three starters, they were swept 3-0 in the ALDS. Their bullpen (I mentioned their terrible bullpen of terrible-ness, right?) totally crapped Games 1 and 2. They were already losing in Game 1, but the bullpen put the game from "behind, but still winnable" to "completely out of reach". Detroit was outscored 21-10 in that series.
Key Offseason Moves
Re-signing Victor Martinez
Coming off quite literally the best offensive season of his career at age 35, Martinez decided to stay with Detroit for a 4 year, $68M price tag. Martinez won't have a 166 wRC+ every season in that contract, so it'll probably be a rough contract very soon. Martinez had a spike in his HR/FB rate, he walked more than he struck out, and he popped us less than his career average. It just doesn't seem sustainable to me. However, a 130-140 wRC+ is still something the Tigers need at DH.
Trading Rick Porcello for Yoenis Cespedes
Porcello is in his last year before free agency, and the Tigers wanted to clear some payroll, so here they are (although the payroll difference is tiny). The Tigers will now run out an outfield of Cespedes in left, J.D. Martinez in right, and a Rajai Davis/Anthony Gose platoon in center. That's not a bad outfield, but the pitching suffers because of it in a way. While they had to give up Porcello, that outfield is probably better defensively than last year's. Cespedes is in his last year before free agency, so perhaps it's just a wash. A swapping of roles.
Trading for Shane Greene in a three-team deal
The Tigers traded Robbie Ray and Domingo Leyba to the Diamondbacks, who traded Didi Gregorius to the Yankees, who traded Shane Greene to the Tigers. Greene will take the position in the starting rotation vacated by Porcello. Greene had a very nice debut with the Yankees, and he's a relatively young, cost-controlled piece. A decent trade, really.
One to Watch
The one to watch is Nick Castellanos. He was a decent prospect coming up through the Tigers' system, and he's a young player at third base. He just recently turned 23. He didn't have a great debut season last year - the defensive metrics hated him, and he didn't do enough on offense to make up for it. However, all the offensive projections on FanGraphs' player page have above average wRC+ numbers with improved defensive numbers. If Castellanos can do a bit better with the routine plays, he could be serviceable at third base with a decent bat. That's not a star, but that's a cost-controlled long-term piece to help fill out the roster around the David Prices and Miguel Cabreras of the team.
Tigers by the Numbers
The Tigers had an unequivocally bad defense last year. They ranked near the bottom in both DRS and UZR. However, they have made some changes. Torii Hunter won't be patrolling the outfield anymore. Cespedes, who overall appears to be an OK fielder with a literal cannon rocket attached to his arm, is now there. J.D. Martinez was slightly below average in his small sample, so I don't have any reason to believe he's significantly different than average right now. Anthony Gose is supposed to be a solid center fielder, and he's the larger half of the platoon.
On the infield, the Tigers remain mostly the same except for shortstop. If Jose Iglesias can stay healthy, he'll be a massive upgrade over the cavalcade of players trotted out there last year. A middle infield of Iglesias and Kinsler combined with Anthony Gose in center gives the Tigers a proverbial "strong up the middle" quality. The Tigers defense will be improved. The question is how much?
2015 Team Outlook
The Tigers got worse. They lost Scherzer and Porcello relative to the end of 2014 and replaced them with Shane Greene and Alfredo Simon.
The comparison perhaps isn't that simple, given that David Price kind of replaces Max Scherzer, but there is a talent dropoff from Porcello/Smyly (who was also traded last year) to Greene/Simon. Everybody else aged a year. Aging a year might be good for Castellanos, but it certainly isn't good for Anibal Sanchez, Verlander, Cabrera, and Victor Martinez.
Overall, their core players are still good enough this year to make the playoffs. Unfortunately, you'll notice that the bullpen didn't appear in the "Key Moves" section. The bullpen is still as it was. A fantastic rotation and offense could cover that weakness fairly easily, but those talent pools are a little bit shallower this year. The Tigers obviously have every chance to win the division, but it's going to be tougher than last year - even though they won the division by only one win last year.