An easy way to create drama on television is to subvert the viewers' expectations. After so many episodes, most people have a good idea what's coming, especially when it comes to Castle. Creating a completely unexpected event can be cheap, but none cheaper than what happened on Castle. Unfortunately, the writers still have not figured this out. When they have a gun pointed at Castle's head, everyone knows nothing will happen, but the writers linger on this forever, trying to get any tension out of the moment. But from the beginning, without the possibility of Castle getting shot or something weird happening, there was no tension. The explosion was probably the only thing that was unexpected, because it happened in the midst of other events and wasn't telegraphed completely.
I liked that the plot went beyond the normal "take the cash" scheme, but the eventual conclusion--the mastermind using the heist to find his ex-wife and son--seemed awfully elaborate. I guess some people will go to super crazy lengths to get their children back. With that kind of planning, though, you'd think he could hatch a plan that wouldn't require drawing so much attention.