I'll have more detailed analysis for each player at Rotowire.com, but I like the Matt Holliday deal for both teams.Holliday is a very good player, but he's not a long-term asset for a team in Oakland's position, and I think they got a good haul in the deal.
Brett Wallace has adapted well to the high minors just one year out of college, hitting .293/.346/.423 this year for Triple-A Memphis. His strike zone judgment has actually been a bit worse than expected and he hasn't fully unleashed his power, but I think that will come. Although I'm more optimistic than some about Wallace's ability to play third base in the short run, Oakland can simply move him to first base now to replace or supplement the disappointing Daric Barton. If the Cardinals concluded that Wallace couldn't play third, obviously they had no room for him behind Albert Pujols.
Peterson is hitting a combined .295/.361/.423 with 12 steals between Class A Palm Beach and Double-A Springfield. I see him as an effective line drive hitter who can play the outfield and first base if needed, though he won't have big power. He should make a nice fourth outfielder.
Mortensen gets lots of ground balls and throws strikes, though he's not overpowering and his 4.37 ERA with a 82/34 K/BB in 105 innings for Memphis is decent rather than excellent. He could make a nice fourth starter, though.
So, in exchange for a 29 year old pending free agent outfielder, Oakland gets a top third base/first base prospect, a guy who should be a good fourth outfielder, and a guy who could be a useful inning-eater. That's a better than what they would get would likely get from the draft picks if they'd kept Holliday and let him leave as a free agent.
From the Cardinals perspective, you exchange some of your surplus in corner bats and pick up a guy who is ready to help you win now. So basically I think this deal is good for both teams in the short run, though in the long run the balance should help Oakland more.