on the upside we could have things like advanced gravity for spherical planets on spacebuild, along with better lighting and rendering,air resistance and other things but, im concerned about these
no dx8 might stop a minority of players (im fine, most people will think this is silly but it will happen)
the textures might get completely changed so everything comes up weird (pong and stuff)
the lua may become broke or unsuported
hl2 npcs may get more pollygons, so the skins of them may not work, same with other models/weapons
new physics might stop stuff from flying/driving well
maybe no gcf files, so things may have difficulty getting ported