Wormholes: PHost supports wormholes. HOST needs an external add-on for that.
Wraparound Map: PHost implements a wraparound map in its core, supporting all features across borders smoothly. HOST needs an external add-on for that. With the most popular one, Sphere, things like Sensor Sweep will not work across the border. There is a rather new add-on, Zeus, which claims to support seamless missions across the map borders for HOST.
Remote Control: In PHost, players can take control over each other's ships.
Player/Race assignments: In PHost, every player can be assigned every race. There also exists a HOST version which does that (SRACE), but it doesn't seem to be up-to-date. PHost also has ways to define completely new standard races.
UTILx.DAT: PHost writes utility data files which can be used by client-side programs. HOST does not, so clients have to rely on message scanning.
Language support: In PHost, every player can choose his language for messages he receives. HOST allows only to replace the language file for all players at once. This HOST feature is rarely used because it breaks message scanning.
Command messages: Some functions of PHost are accessed using command messages. A command message is a normal sub-space message addressed to yourself. HOST does not treat such messages specially.
Giving away Planets: PHost lets you give away planets as well as ships. In HOST, you can only give away ships.
