It's kind of neat when a villain shows up in OP that just can't be beaten (whether it be at that point in time or just an overall unfavorable devil-fruit matchup). So far they've done it with...

1) Mihawk (who left of his own volition)
2) Smoker (who gets intercepted first by Dragon and then by Ace)
3) The Ice guy (who left of his own volition)
4) and now Kuma

I kinda prefer it when the main character isn't capable of defeating all the villains himself though.