My guess is that his success in the majors will be determined by whether he can continue to get the ball over the plate. Also working in his favor, however, is the ability to induce ground balls--he usually (except at AAA last year) has had ground ball rates of at least 50%, and his career average of 0.48 hr/9 in the minor leagues is excellent.
It will be interesting to see how long Salmon can stick with the Reds this year. Jared Burton's rehab assignment is likely to expire within the next few weeks. Unless Salmon can really impress in his first few appearances, my guess is that he'll return to Louisville when the Reds are forced to promote Burton again. And even if he survives that transaction, the imminent return of Gary Majewski and eventual return of Bill Bray may also force him back down the river.
Still, I think there's a chance that Salmon can help the Reds this season. As Narron pointed out, they don't have much in the way of power arms in their bullpen outside of Coffey, and he seems to be pitching in the low-90's rather than the high-90's that he very occasionally flashes. If Salmon can be effective (throw strikes), he'll provide a nice change of pace from the other right-handers in the bullpen. According to PECOTA, one of Salmon's comparable players this season is Derrick Turnbow of 2005 (1.74 ERA, 67.3 IP, 64/24 K/BB)--obviously a best-case scenario, but that would be a nice addition to the 'pen. :)