Two of the major shortcomings of ZigBee and Bluetooth (that I know of) are that they don't handle congested scenarios well and there's no standard handling for priority interrupting other transmitters. BT and ZigBee work well when there's a couple (like 2 to 5) 2.4GHz devices in a broadcast domain. If there's more than that (active nodes) then they'll commonly cause problems. At the time those standards were written they were a replacement for desktop peripheral cord, BT in particular takes a PtP approach to the connection.
The new BAN standards will address what happens when you have 10 patients in close proximity (broadcast domain), each with a dozen sensors, all sending data constantly (or consistently enough that it might otherwise cause problems with existing standards). And also address how a sensor with an alert condition might guarantee throughput so that it's condition can be reported (can you imagine an arrhythmia going unreported while a senor contends for bandwidth in a congested environment).
This is by no means an exhaustive list of what needs to be addressed, other feel free to add what you have heard. But I can see a few issues that need to be dealt with.