My best explanation to why this is a very good card is this, people love plus power and strength charms; they allow us to do more damage with our attacks.
Lake Boundary is like a intensified, focused boost for attacks. If you are lucky enough to run into a Pokemon that you have it's weakness, Lake Boundary can double your damage to them. This is so important since the release of DP. HP's have gone up, and a lot of the DP Pokemon do more damage, but the pre-DP cards don't get those upgrades and it's hard for the older cards to compete with the newer cards because of their lack of upgrades. With Lake Boundary, the older Pokemon have a way to do the big damage needed to KO the bigger DP Pokemon quicker. Not only that, but it allows the pre-DP Pokemon to make the field more equal, making both Pokemon 2x weak.
Now there is some means to abuse this card by pre-DP Pokemon that have no weakness, and I think it's a good way to use it. But I think we will see this card used more often to even the sides and to take out metagame'd threats easier.
I don't know if I could rate this card. It's usefulness varies from deck to deck. There are multiple ways to use it, and many ways it would be useless.