Firstly, there's 1B Chris Carter, who was a Diamondbacks for exactly 11 days at the end of 2007. As you may recall, he was what we got back from the White Sox in exchange for Carlos Quentin [deadhorse,gif], but was then rapidly dealt over to Oakland in exchange for Dan Haren. He hasn't seen a great deal of playing time in Oakland, and hasn't exactly done much with it either, hitting .214 in 106 games, though has shown power, hitting 19 home-runs for an OPS+ of 102. Sixteen of those long-balls were in only 218 at-bats last season, so he may be about to fulfill his potential. He'll probably see some DH work for Houston, along with time at first.
Secondly, it gives the A's an everyday shortstop to replace Stephen Drew, who will be over on the East coast, playing for the Red Sox in 2013, which is going to seem very, very strange to me. I was quite surprised to realize how old Jed Lowrie is: he'll turn 29 a couple of weeks after Opening Day., so he's only about a year younger than Drew. Though this will be his sixth season in the majors, he has yet to play 100 games in a campaign. The failure to do so in 2012 wasn't his fault, however, as he suffered a leg injury when a base-runner slid into him as he covered second-base, which lost him about two months in the second-half.
But perhaps the most interesting thing is it does show the market for shortstops does seem to be a seller's one at the present time. Lowrie is not a long-term solution for Oakland, since he will be a free agent at the end of the 2014 season. And he had not even been all that good, until his breakout season last year, putting up around three WAR in 256 games with Boston, before they traded him to Houston as part of the Mark Melancon trade. If the Athletics get the 2012 version of Lowrie, and if he stats healthy, then he should be a productive part of a team which is looking to contend in the AL West. Neither of those are guaranteed.
The odd thing is that Oakland put themselves in the position of needing a shortstop, by trading Cliff Pennington to the Diamondbacks. Indeed, there's a case to be made that they'd have been better off sticking with what they had (at least in terms of a shortstop). They're basically the same age - Pennington is a couple of months younger, but has a year less service time, so is under control for longer. Career batting averages to date are almost identical: Pennington has hit .249, Lowrie .250. Pennington has been more valuable (fWAR 7.3, bWAR 5.6) than Lowrie (6.1 and 4.6), though has played in considerably more game, so in rate terms, Lowrie has the edge.
Admittedly, they aren't identical in other ways, and how you value them would depend on how much weight you put on their differences: Lowrie has considerably more power, with a slugging percentage over 100 points higher. However, Pennington seems currently to be the better defender at shortstop, with a UZR over the past three years of 10.7, while Lowrie is at -6.8, and has been below zero each of those seasons. But I do feel we gave Oakland less for Pennington - a one-year rental - than they gave the Astros for Lowrie, including a potential long-term starter at first-base, as well as a couple of other prospects.