Dethrone is usually easy to trigger, and even without it, you can use other +1/+1 counter effects to make sure your creatures keep coming back
Given Weezing's rise in prominence and the supporters added in Mythical Island, Blue stands out as one of the best options
The card was immediately identified as a potentially problematic combo engine, thanks to the interaction with the cascade mechanic
Finally, Thrasios, Triton Hero isn't exciting, but it's not really a bust either, essentially being a repeatable (but expensive) Coiling Oracle when it's on the battlefield
They all had the same type of cut (like the cloyster above), all were water type, all were from breakpoint, it all lined up
In terms of design, Manaphy is inspired by marine life, featuring a blue, jellyfish-like body with large, expressive eyes that endear it to many fans