Well, at his best, he was an utter monolith - able to tear attacks apart with the bat, able to bowl his heart out and incur crucial breakthroughs and able to field adeptly in the slips (and even on the outfield). His mere presence could inspire lesser lights in the team. His performance during the 2005 Ashes, much as I hate to admit it, was close to perfect (arguably bettered only by Johnson's performance in SA since).
Unfortunately, he wasn't at his best all that often. There were a few reasons for this. For one, he was amongst the most injury-prone cricketers out there. Plus, his batting was flawed - his technique was decent against the quicks, but against spin he was utterly suspect (sometimes failing to pick variations and tending to run down the pitch like a deranged madman). He rarely ran through sides with the ball, either. He looked good hurling it down at a back-of-a-length, was often economical and was a partnership breaker of some renown, but you couldn't reasonably call him a strike bowler. In a team with a more reliable strike bowler, he probably would've done somewhat better - compare his performances nowadays to when Harmison was at his best (during 2004 and some of 2005), for instance.
He probably suffered, like Broad may be now, for being introduced into Test cricket some years before he was ready. He debuted in 1998 and arguably only become a viable Test selection in 2003. Add to that his fitness issues at that time (being too fat) and you can see why his record is still a touch underwhelming.
But anyway, I wish him the best for the future. This is still a loss for England, although not as big as it would've been say, four years ago.
