T.J. Mittelstaedt was a 44th round pick out of college (where he only had one good year of power, but hit for average and OBP consistently) in 2010. However, since signing, he has been pretty successful (named a MiLB.com organizational all-star after the 2011, whatever that means). In the AZL after being drafted, he had an OBP of .383 and then had a .410 OBP in Class A in 2011. This year, he began in advanced A and had a better SLG than in the AZL and Midwest League (though had a similar wOBA +, meaning the league is easier to hit in), but found the jump to AA very difficult, hitting just .189/.318/.344 (so still walks) in 44 games. He was horrible against left-handed pitching, but not bad against righties.
He has is somewhat small for a third baseman at 5-10 185 pounds. He looks really selective against right-handed pitching, but when he faced a lefty he looked absolutely lost. Defensively, he looks somewhat athletic. The arm has some problems at 3rd base as he goes straight sidearm and doesn't appear to have a lot of strength in it.
Because he played 4 years of college baseball (will turn 25 before the season starts), his development in the minors has to accelerate faster than most minor leaguers. It isn't a surprise that he was successful in the AZL or class A since he was older than his competition. This makes 2013 a big season for him, as he really needs to have a good year in AA and even get to AAA by the end of the year.
Adam Weisenburger is another low draft pick (34th round in 2011) after 4 years at the University of Miami (in which he was an okay but not good hitter). The catcher didn't hit for really any power in the Arizona League in 2011, but walked enough and had a good OBP (with more strikeouts than walks). He spent 2012 between A+ and AA where he struggled mightily with the bat.
He looks pretty selective, not chasing pitches out of the zone. However, it is not a very good swing on outside pitches, though he has a decent looking contact tool. Weisenburger had a nice hit the other way when I saw him.
He seems like a good receiver behind the plate but didn't show good blocking mechanics, instead trying to stab at the ball. His arm looks really solid and he seemed to get rid of it pretty quickly.
Jimmy Nelson is a right-handed starter for the Brewers AA club with pretty easy looking mechanics. He was the club's 2nd round pick in 2010 after playing with the University Alabama, and has been really good in the minors ever since. When I saw him, he was around 92-93 MPH with movement and a decent slider. He is a big guy and the command is not bad, but wasn't sharp when I saw him. His curve is not much different than the slider except in placement. He throws the curve inside to righties usually and it is thrown a lot less than the slider that he threw away (with a little bit of two plane break) from righties (and it was much more effective).