Deolis Guerra is almost all youth and projection at this point, with his one major current skill an above-average changeup. His velocity is fringe-average and wildly inconsistent within outings, so the optimistic projection is that he'll eventually sit 92+, but there's also a good chance that he settles in where he is now. His curve has some depth, but like his velocity is inconsistent, and his feel for the pitch is particularly poor. He has a great pitcher's frame, 6-foot-5 with very broad shoulders and lots of room to fill out, and the fact that he has an advanced secondary pitch bodes well for him improving his command and feel with experience. But the bottom line is that he's 18 years old, so in the best-case scenario, he'll be contributing in the majors in three or four years, and that's if he doesn't get hurt or hit a snag in his development.
The other two arms are major-league ready or close to it, but have limited upside. Philip Humber's curveball was once a big-league out pitch, but since Tommy John surgery, it hasn't been as tight and he hasn't commanded it as well, leaving him more reliant on his average fastball to get hitters out. He has good control and an average third pitch in his changeup, but his days of upside as a No. 2 starter are gone. Kevin Mulvey also has a three-pitch repertoire but lacks any kind of out pitch, making up for it with good command and a willingness to mix his pitches up to get hitters out. Humber has a chance to break out of the 4/5 pitcher group in Minnesota, which also includes Glen Perkins and Kevin Slowey, while Mulvey just gives them another cheap option if one of those guys isn't ready in April.
In the abstract, it's hard to accept dealing your marquee player and top trading asset without getting your partner's top young player in return, and that's what the Twins did. They did get back significant economic value in four young players, each of whom has under one year of big-league service and two of whom aren't even on the Mets' 40-man roster yet, so the Twins will have each of them under control for six full years of service. That return in exchange for just one year of Santana's services is reasonable. But premium players should fetch premium prices, because there's value to a club in having so much production coming from a single roster spot. And in this case, Minnesota GM Bill Smith did not get a premium prospect in return.