It's funny that there was some time when I thought about implementing something similar for my campaigns. It's an interesting system, but at the time there were several things that made me reject it. For instance, spectres and level draining creatures suddenly become far less dangerous. It would also be almost impossible to die from level draining: a character would die sooner from the small wounds that these monsters also cause. Also, constitution bonuses would either have to apply to the dodge bonuses (which doesn't make much sense) or to the body pool... and since they can be as high as +4 per level, that would end up in having more or less the same problem as before. How did you address these issues?
People might call me crazy, but I think the best of the versions was the first -not even the advanced (the series of Basic, Expert, Companion, Master and Immortal): the very first. It was a complete and balanced game, with the best immortal rules I've seen so far. I actually thought about using it with the additions I saw fit, though I never had time to actually doing it. I like the idea of having characters have different progression speeds with the wizard as the ultimate power... if he survives which should be very difficult. That was lost I think in other versions.
With respect to the third edition, I don't like the idea of having hundreds of unbalanced and in some cases ridiculous "prestige clasess". It's just an excuse to fill more and more manuals. I hate that someone decided that necromancy should be reserved mainly for clerics (!!). I resent that there are loads of cleric spells that are mere copies of former wizard ones. They even have miracle which is nothing less than the once legendary wish! Currently a priest is a wizard with armor, more hitpoints, capable of healing and that does not requiere to use a book of spells... I however like the attacks of opportunity system, though I never use figures. I prefer to keep the images in the mind of the players. Placing a figure kills it, and also destroys the idea of a chaotic and moving combat. Excuse the long rant...
With respect to critical tables I played Lord of the Rings -which uses the same system Dark Russel describes- and I find them risky (which is not necessarily bad, although needs to be taken into account) and incomplete. You would need a full book of tables to make that system coherent: how can a goblin crush the skull of a dragon?, for instance. What if you fight a jelly or a black pudding? Or something with no organs? Or something with no certain organs? What if someone is fighting a monster that is below him?Would he still be able to sever his leg, for instance? I found it funny... but unbalancing and difficult to really make it work. However it's just my personal impression.