What lies in the room is PROBABLY (not sure if it is confirmed or not) "something that is at the same time more terrible and wonderful than death".
I believe (Although I could be mistaken) that, somewhere back in the land of Harry Potter and the Philosopher's Stone, Dumbledore said "Many men have wasted away in front of this mirror. While it is wonderful to see what we dearly desire, it does not do to dwell on dreams" or something like that, about a certain Mirror of Erised.
I sat there after PotterCast last week thinking...What if, and this is just my theory, in this locked room is the Mirror and that when we go there in the seventh book, Voldemort will see that what his deepest desire is, is to be loved by someone. That will be his downfall and Harry will defeat him there and then.