I think Wailord is pretty nice. 200 HP is great, currently there is no attack in the game capable of an OHKO without at least attempting to add Weakness (only thing I can think of right now that can OHKO is Electivire SW with 4 Energy, flipping all Heads). For most Pokemon, it's a 3HKO that'll require a sacrifice to KO (unless you have 130+ HP and aren't weak to Water so you can take 2 potential hits from Wailord); 2HKOs are possible, but few Pokemon can churn out 100 damage consistently (not named Gallade blowing them to the bench with 50 HP left btw), and even then they'll be left weakened by 60-120 damage from Wailord. And of course, all of this is not taking into account that Wailord potentially recovers 10 HP each turn if it attacks with damage counters on it. Imagine combining this with Leftovers, lol...
Which of course, brings us to Wailord's best abilities, as a staller. He is awfully big for a Stage 1, and like I mentioned before, could potentially recover 20 HP each turn just with his Poke-Body + Leftovers. This, along with Potion, Buffer Piece, and such easily allows Wailord at the least to be out there 3 turns, possibly more; this is more than enough time to bring up another Wailord. And like I saw in a deck posted somewhere here, a simple tech of Xatu SW will remove Sleep when it comes to your turn if you're still afflicted by it and force it onto the opponent, while you're free to attack for 60, go to Sleep (if you're damaged still), and recover more HP.
About the only factor against Wailord are his high attack cost (3 Waters and 1 Colorless is not cheap, and at 60 damage is underpowered) and his high Retreat Cost (you aren't retreating this guy usually though, but if you have to have him out early, ouch...Warp Point and Switch help a bit).
So overall, I think Wailord is pretty good, but I doubt it'll be gamebreaking. Might win a random few Spring BRs, but I doubt it'll be winning Nationals and such unless the metagame shifts in such a way as to allowing him to dominate.