I've been reading a Passenger Survey website and several have commented adversely on the practice of allotting boarding group D to those pax who opt for online check-in.
They said they were not aware this would happen (as far as I can determine easyJet don't mention this practice in their web press release on the subject).
These people seem to be implying that those who check in online should be in some way rewarded by being given a priority when it comes to boarding.
If this were to happen and everyone began to check in online then easyJet would finish up with a Ryanair-type free-for-all because everyone would have the same high priority status.