A nickel corner needs to be quick and aggressive...a real dog in coverage...because the nickel covers the slot WR, the likes of Deion Branch, the best slot WR in the NFC West.
The problem with covering the slot is that the slot WR is not lined up on the l.o.s. and therefore is too difficult to bump and run...the slot also gets motioned a lot, so the nickel CB has to motion with him and still try to break with WR if he runs an out...or keep inside the WR to take away the slant...and run step for step with the WR on drag patterns, seams, z-outs and gos.
Antrel Rolle is a sitting duck in the slot...he can't bump and run because the slot is off the ball, and then he lacks the quickness and speed to shadow the speedy slot WRs like Branch.
Glenn is still quick, plays near perfect angles, breaks on the ball well and has great anticipation of where the ball's going (a.k.a. savviness).
Evan Ogelsby played well in the nickel in Baltimore...but ironically the Cowboys claimed him after releasing Glenn. The Cards really missed the boat on that one.
The other guy I liked was David Jones (Saints) but he was claimed.
If you notice, not too many cover/nickel CBs were waived...they are a hot commodity...which is why many of us have been upset that the position was not addressed at all in the draft, nor was another key free agent added.