This looks like a great monster concept; my worry is that WotC won't do the work to rationalize how its abilities work in combat, in a way that stands up to player scrutiny.
It's kind of a blind spot WotC has. They'll go to extravagant lengths in every other regard to make the DM's life easier. But then they put in mechanics like the picador harpoon--mechanics which will cause major suspension-of-disbelief issues without a carefully-thought-out explanation--and slap on a shaky bit of fluff that even a mildly inquisitive player will tear apart.
My criticism above is one I absolutely expect to hear if I use this monster in combat. The picador will throw his harpoon, snag some big strong PC, and the first words out of that player's mouth will be, "Okay, I drag him along. He's a goblin, how the heck is he going to hold me in place?" It's a totally rational and logical response.
Now, I can think up explanations that will cover this situation. Maybe the picador is crazy strong for a goblin (he is, in fact), and he grabs onto a nearby terrain feature and hangs on like grim death. Or maybe once he's harpooned you, he knows how to keep you off balance so you can't set yourself to pull away.
Either of these could work, but I don't want to have to stop and think them out mid-combat while the game comes to a screeching halt. And not every DM is as good at rationalization as I am. It would be really nice if WotC would put the same effort into the interface between mechanics and game world that they do into the mechanics themselves; that interface is, after all, where the rubber meets the road. WotC's game systems tend to be like cars with massively powerful, perfectly machined engines, which are sitting on four flat tires.
Of course, we haven't seen the full monster entry, and I might be wrong. Maybe WotC has come up with a really clear, well-considered, robust explanation for the picador's abilities. But, based on past experience... I kind of doubt it.