I think Glen Johnson's a great player, there aren't many better right backs in the league, if any at all.
Having said all that the one thing that winds me up about him is his concentration. He's such a good player going forward, with excellent technique and dribbling ability for a right back that gets him into good positions on a regular basis. Too often though, he'll get into these positions and then play a sloppy final pass or fuck it up some how by lazily (or at least it appears that way to me) executing what should be the simple bit.
I actually rate him defensively as well, but he switches off there at times too. Recently this has spread into his general play, he's been letting the ball roll under his foot, misplacing easy passes, though I'm sure that's just a bit of bad form and he'll be fine.
I don't want to replace him or anything like that, he's usually boss, but if he could just concentrate for whole games he'd be even better imo.