Some very important technique like as BSP portal , PVS set... too old date, I do'nt hope discover in Half Life 2 engine, Because , Engine can't has good performance ,If it not has 3D hardware ,These are my view point.
bsp is the most efficient scene organize method so far. if u dont use hidden-thing remove algorithm like bsp & pvs & lod, even with nowaday's video card the game will be slow like hell. well no one like that dont they? or u have some much intelligent ideas?
and IMHO they dont have quite a lot to do with 3d hardware.