Adam Eaton is not very good. He has a below average career FIP and this is now the tail end of that career. His velocity was never plus and in an up and down '08, it tracked 90-91. In his last start, he wasn't breaking 90 with any consistency. It doesn't rate well in the run/rise department either, so getting whiffs or ground balls on movement/speed alone isn't going to happen.
The pitch f/x data says that his fastball works better against lefties than righties in terms of ISO, but I think that's more than likely a blip in an incomplete data set. PECOTA agrees:
vs. LHB: .309/.400/.506
vs. RHB: .282/.349/.445
There's just nothing special about the heater. The offspeed stuff is decent and they all likely play up because of the variety Eaton throws. Righties get mostly fastball/slider, but he also mixes in a change and curve. He has problems throwing the curve for a strike and on top of that, he'll hang one every now and then too. The slider is average-ish, but he misses with it more than most, likely thanks to excess nibbling. He needs hitters to chase to be successful, so there are walks to be had, especially for power hitters.
Against lefties, he mixes his offspeed fairly equally, but none of them qualify for out pitch status. It's easy to see why PECOTA thinks lefties will go nuts against Eaton in '09: he doesn't have any consistent way to get them out. He doesn't throw enough strikes, nor can he get whiffs or ground balls. Unless Sox hitters are helping him out and expanding the zone, they should be good for at least 5 runs today.