There's a very kind feature on Andrew McCutchen over at Yahoo that describes his potential as "[Brewers outfielder] Corey Hart with more speed." The feature is for fantasy baseball, and for fantasy, the Hart comparison is probably fairly accurate, in that McCutchen will hopefully rack up good averages, stolen bases, and solid but unspectacular power. Also like Hart, McCutchen is a plus fielder. But McCutchen will hopefully also have much better strike zone judgement than Hart has shown--Hart had 27 walks against 109 strikeouts last year, and I doubt McCutchen will ever have less than one walk for every two strikeouts, except maybe in his rookie season. Ultimately, I think McCutchen will end up with a bit less power than Hart, but better on-base percentages--Hart's career mark is .323, and I think McCutchen could probably top that right now.
PECOTA (subscription required) doesn't see McCutchen as a sure thing quite yet, but it compares him to several very valuable players whose home run power peaked at about 20 per year--Shannon Stewart, Chuck Knoblauch, Chet Lemon, Mark Kotsay. (Another comp is, remarkably, walk machine Eddie Yost.) I think Hart will eventually have a year when he hits 30 or 35, but I doubt McCutchen will. McCutchen could end up posting .380 OBPs, though--Lemon did that three times, and Knoblauch did it six times.