"Now onto HL2. Valve boasts a completely new engine built from scratch, which means that the common MDL format that all HL models use now will likely be scrapped. The MDL format was taken from Quake, since the original HL engine is based on a modified version of the Quake 1 & 2 engines. Hopefully a 3D Studio Max plug in will be released so that the modeling community won't suffer the slow down it did at the beginning of Half-life's release."
Nice little read if you are a modeler.