One thing to keep in mind when judging Reyes is that he's only started 28 regular season games. Is this a sufficient number to start making conclusions about his likely future? To help answer that I thought I'd look at what other notable Cardinal hurlers did in there first 28 starts:
Pitcher IP Hits/9 HR/9 BB/9 K/9 ERA
Reyes 147 8.8 1.5 3.4 7.3 5.45
Benes 161.2 9.5 1.3 3.9 6.5 5.40
Haren 148.1 10.3 1.1 3.5 6.3 5.28
Ankiel 151.1 7.7 1.1 5.1 10.0 4.16
Gibson 186 9.2 0.6 4.7 6.6 3.97
Magrane 180.1 8.4 0.4 3.1 5.3 3.34
Morris 179.1 8.5 0.5 2.9 6.3 2.91
This just reinforces what we already knew: Reyes' peripherals are quite good, but he gives up too many long balls and, while not outlined here, he crumbles with men on base.
What also jumps out from this data is that all of these pitchers had very different career tracks. Gibby and Haren obviously had slow starts to their careers but picked up steam later. Benes went down with an arm injury and never came back. Morris also went down, but came back as a somewhat different pitcher. Ankiel is a unique case. Magrane was a Mark Mulder type with Mulder's career track.
Reyes' K to BB rate is the best on the board, and his hit rate is good. Obviously he needs to work on keeping his fastball down and off the middle of the plate, but as this data shows, almost all of these eventually good (or great) pitchers were unfinished products after 28 starts.