I think prog death has a lot more different influences, like jazz, blues even, so they sound very apart from other death metal bands. Also consider the many tempo-changes, clean passages, etc. Technical death is very similar (if not the same) as brutal death, in that it is much faster and there is a lot of alternative picking, sweep-picking and arpeggios, as compared to 'conventional' death which is slower and uses powerchords and inverted powerchords with some alt-picking and arpeggios.
I'm not quite sure myself, my definition is based on what I hear when comparing Nile, Krisiun and Necrophagist and comparing with early Dying Fetus, Obituary and Malevolent Creation.