The NIV 2011's translator notes explain a number of their translation decisions. As I said in my initial reaction, I like a lot of their decisions about so-called gender-inclusive translation issues, even if I don't think they quite got things right.
One problem is that they avoided using the very singular "they" that their own research determined to be pretty much standard English nowadays except if it was ridiculously awkward to avoid it. As a result, they did end up with some not ridiculously awkward but still nonetheless awkward constructions just to avoid standard English. It's much more awkward to translate a personal pronoun as "the one who" or "the person who" when the alternative is simply "they".
I do appreciate their attempt not to create ambiguities with plural-form pronouns with singular reference. It would be bad to have a singular-referring "they" in a context where it's not clear if it is singular-referring (when the original language has no such ambiguity). That's one of the problems with the TNIV that they're trying to correct (and the NLT and NRSV demonstrate similar problems at points).
They chose to avoid singular-referring plural-form pronouns unless there's a singular antecedent in the context that can remove the ambiguity by serving as the most obvious antecedent. That strikes me as a good decision. Consider the examples they give:
Whoever has will be given more; whoever does not have, even what they have will be taken from them. (Mark 4:25)
It's hard to hear the "they" there as referring to a group. It refers to any individual who does not have, who will even lose what they have. On the other hand, consider the following two statements:
When Jesus comes into someone's life, he is a new creation.
When Jesus comes into someone's life, they are a new creation.
The first statement is ambiguous, if it's appropriate to use "he" to refer to anyone regardless of their gender. But it doesn't actually come across that way, at least to me. I cannot read that statement as a good paraphrase of "anyone in Christ is a new creation". The "he" seems to me to have to refer to Jesus being a new creation. It just doesn't seem natural for it to be referring back to "someone". The second statement removes the ambiguity in a way that makes it impossible to take "they" to be referring to a group.
Consider also something like:
Everyone who is righteous will receive his reward.
I read that, and I think, "whose reward?" It seems to lack an antecedent. I just can't hear that sentence with "his" referring to "everyone" in the subject. I have to work very hard to make that meaning come out of the sentence, and it would require the same double-take it I encountered such a sentence in the writings of someone who died over a century ago who would have used such a statement in a much more natural way. There are people for whom this is natural, who just don't interact with the mainstream population and thus don't understand that "everyone who is righteous will receive their reward" is much more natural for most English speakers.
Combine that with the fact that the committee looked to hard data to see what actually is being used, and I think they've got some good support behind their decisions on this question. Instead of arriving at a translation result politically by assuming an ideology (whether feminist, traditionalist, or whatever else it might be), they decided to be translators and translate into the actual English language rather than English as they'd like it to be. That's how they ended up with "mankind", which I would oppose if I had a vote. But apparently it's still common enough, whereas using "him" to refer to a gender-unknown person isn't really common anymore but common enough that they left it in a number of places when the alternatives were just too awkward, ambiguous, or problematic in whatever way.
So there's a sense in which what they're doing is simply the insistence of Protestants in Bible translation since the Reformation (and most Catholics since the Counter-Reformation). They seek to translate into the language as it's actually used, and they went to an actual study providing hard data rather than going with their gut about which forms people actually use. You might prefer that the language not change in the way it's changing or that it change more rapidly, but they're Bible translators, not language architects. They're not in this in order to figure out how the language should be changing and how rapidly. They're simply trying to put the Bible into the language most people in fact do use. Given that criterion, several translation choices that are controversial (and in my view worth avoiding if you do want to move the language in the right direction or prevent it from going in the wrong direction) suddenly become the most appropriate choices, in a way that both sides of the broader debate will quickly lose much effect from their arguments, since those often have to do with maintaining or resisting some kind of status quo about language-use. If that goal is no longer legitimate, then all you have left to do is translate into forms people actually use.