Setting indicators is difficult. It is especially difficult setting indicators attached to incentives as they are in the QOF. It is especially difficult to produce indicators that actually produce the result that you are after without producing perverse incentives.
Required reading for anyone setting a target at any level, from practice to nationwide, should be the very readable report from the King's Fund report Getting the Measure of Quality. It clearly documents the successes and potential failures of information gathering and some of the possible remedies. It would certainly be interesting to see some of these standards applied to QOF indicators although there are signs that the new NICE committee is thinking along broadly similar lines.