This is the first that I've had time to actually sit down and look at code, the RFC, etc, and I'm scratching my head.
The rationale for the payload for DTLS (sent over UDP) is reasonable, but I'm scratching my head over why the payload is even there for TLS. I guess there's some logic in making the protocol the same irrespective of underlying transport, but the payload is completely redundant for reliable protocols, which will handle any retransmission or reordering automatically.
Edit: Upon further (amateur and arguably ill-informed) reflection I'd argue that the heartbeat functionality shouldn't apply to TLS at all. It's needless complexity that can be handled by the underlying reliable transport. That doesn't change that the OpenSSL project clearly has code-quality issues, but it would have averted this particular fiasco, at least.
I am dismayed that a security-oriented protocol isn't focused on minimalast design. That seems very backward to me.