Implicit vs. explicit interactions/contracts. Both between consumers & producers but also...

The features, constraints, goals, etc. expected out of the ESB substrate by the different use cases becomes a nightmare of responsibility/blame shifting. Durability and replay? That's the ESB's problem. Load balancing? Yep, ESB. But it's screwing up the ordering guarantees! Well, make the ESB 'smarter' and we can just keep punting all of our problems to someone else. Etc.