Actually, it was the wars against France during his reign that put the financial strain on the kingdom. Lost quite a bit of the empire the kings of England had made from themselves--well, because the kings of England and the kings of France are related! ... not sure how, but! In any case, beyond that aspect, John actually had a fair bit of positive going for him in the way of political reform of the judicial system--English Common Law, isn't it?
... although John did get into a bit of trouble with the Pope around that time.
And after YET ANOTHER failed battle against the French, the barons of weren't at all pleased with the King. At that point, Magna Carta was created. Mythology says that King John was forced to sign the document, but it was quite the opposite. He was as much for it as anyone--which is to say not at all completely, but still a very important document for him to sign in the terms of history and cause and effect and the eventual parliamentary government England would have.