*Sorry for long reply; I like these discussions, Feel free to skip post*
At first glance it looks OP I agree, and asking about it is a perfectly fair question. However as previously stated it will require A: 2 player coordination, and B: Thresh to be out of the action to use it as an escape ability. So far as bringing an initiator into a fight, I agree it is opening vast opportunity for gank and surprise attack potential. However I'm getting the impression that using Thresh to his full potential will be extremely difficult, and if zoned properly in early laning, not only might he be hurting for gold, and unable to buy items fast enough, but he will be under armoured/MR'd since he won't gain Arm/MR per level. If this happens, focus him before fights to eliminate his role. That and he is (From the look of it) all utility, no damage. So, as with any champion, get to know them so you can understand what to watch out for against them in game. From what I've seen, his lantern might be very easy to anticipate. If it gets tossed out where you can't see, back off. If he tosses it into a team fight, instead of focusing the target (depending on situation) immediately make course for Thresh, zone the pull and make it illogical for his ally to use the lantern. However, if he isn't there, neither is his CC, and if he gets an ally out, maybe you miss out on 1 almost-kill, but there will be a number of seconds after the pull where you may very well be fighting a 5v3 fight. So if Thresh makes a pull, hammer on the pressure and shutdown his chance to re-engage the fight. These are just my thoughts. To sum it up though it looks like Thresh's kit as a whole will primarily Punish the Greedy and Encourage enemy to be united and uniform in their engagements, if you can avoid impulsive actions and solo missions, I'm sure Thresh will find himself on the same level as everyone else... Or just get banned in every draft game =P
Some Comparisons that came to mind:
Although Lantern's distance is greater, it doesn't do any damage, and requires two people so I don't see it more OP than:
Ezreal: Arcane Shift
Kha'Zix: (Evolved) Leap
Jarvan: Dragon Strike-> Demacian Standard Combo
Which are all one person, on demand wall passing re-positioning tools enabling escapes, and ganks with a lot less warning or fore-sight than Thresh's Lantern.
Other mechanics to consider in live, to see how they act with lantern too are:
Nautilus/ Amumu pulls: If the target grips lantern as a pull is engaged on them, will Nautilus or Amumu, also travel towards Thresh?
Vi: Assault and battery: After she's locked on, I've seen her still track to, and execute ultimate on target even after her targets location is drastically shifted. So perhaps she'll be able to invite herself along for the ride.. and something about bringing an enemy Vi into the middle of your team seems.. a bit.. Counterproductive
Anyways thanks for reading!
Just my thoughts on it all,
but man am I stoked to give Thresh a whirl!