Diehard feat, as it is currently implemented, is nearly worthless from level 4 onward (WF essentially get this feat for free anyway).
As it is a 'feat' - I would like to suggest making it more appealing (and more in line with the divergence of DDO from D&D).
In addition to the way it currently is, Diehard should also grant +level in HP that you can go into the negative. This could further be improved by enhancements. In this manner it would at least be a little more useful (for taking a feat slot).
With the current way the feat is, and 10HP really being 'nothing' after the lower levels, the feat just has no worth in DDO (it is a great feat in D&D, because you can recover from a near death, but in DDO you take such massive damage that 10 HP is meaningless).