I think that this is the most important element. The obvious issues with 100% inheritance tax are this, and the lack of point to working once you are reasonably set.
Personally I think that inheritance should be taxed higher than it is, especially large estates. But it's much more important to implement our current tax laws - IHT is basically an optional tax for the super rich, as stuff is just moved out of estates and then it can never be touched. We need a better system for taxing trusts, both money going in and coming out.