Apart from that, the plot was complete tosh and didn't even make sense (for instance see ajr's list here), but it was very enjoyable and engaging. And so much happened. After half an hour my daughter and I were astonished to see how little time had passed for the amount that had happened. And satisfying without being too deep - just like the original.
Now for big spoilers about the final scene.
Some of you might have seen through the subterfuge right away, but for a few seconds I actually thought Watson was Moriarty, and it seemed a terrifically audacious bit of writing: though it would self-limit the series, and I'm sure they wouldn't do that.
Yes, Moriarty was a bloke. I understand the actor Andrew Scott is very well thought of, with loads of awards, and I understand that they were trying to achieve a similar feel to the relaunch of the Master in New Who. But for me the concept just didn't quite work. Scott is the right age (he would have been a young teen in the late eighties) but he just seemed too callow and weak. I think the fault was possibly the direction, or a mismatch between talented actor and role that wasn't properly thought through.