The thing is, I actually don't think Krejci's been playing all that well as yet, by his standards. As you said, his defense has been sketchy - he's found himself caught in no-mans-land in the D zone quite a bit - and offensively we've definitely seen him be sharper and put his outstanding vision and IQ to better use in the past. But such is his talent that even at 80% or whatever number you want to put on it, the guy can produce at over a PPG. Of course it helps immensely that he's usually with Pasta and Hall, but still, you can't see Haula or Coyle putting up quite the same stats in that position.
Of course, this can now go one of two ways. If his current level is all he's got left then that's ok but it's probably not going to quite cut it when the playoffs and better opponents roll around. But if it's only a beginning and there's more to come, then that's going to be proper exciting. I think given he had a full year away from the NHL, we can reasonably hope for option B, but we'll just have to wait and see. Still, even that possibility and just having him around again is very nice.