For example, while the letter makes it clear that if you disagree with the Church that abortion is an unjust taking of innocent human life, you are to refrain from receiving Holy Communion. But if you publicly advocate a legal right to obtain abortions--well, the letter doesn't specify whether you are in communion with the Church or not, even though previous statements from sources have made it clear that such person are not, in point of fact, in communion with the Church. Similarly, the ban on contraception, which, according to Humanae vitae is intrinsically evil, is widely ignored by American Catholics, but the letter does not mention that topic either.
In one sense the letter was a very good idea, because I'm sure it came as something of a surprise to some Catholics to find that, at least in the eyes of the Church, they ought not to be receiving Communion (though I feel quite certain that, in their own eyes, they are perfectly fit to do whatever they please, since who are these stuffy old bishops to be telling people whether they're fit to receive Communion or not?). In another sense, though, the letter clearly lacked bite, and Mike sums things up rather nicely this way:
Once again, the ball has been dropped. I suppose I shouldn't be surprised: even if this weren't just business as usual, the bishops' "street cred" is rather low right now due to the sex-abuse scandals. But it would have been refreshing, even evangelically energizing, to see some courage. Not a few non-Catholic Christians are scandalized by how Catholics can thumb their noses at the Church with impunity.