Quake 1.5 is a two part retro visual overhaul for Quake. The first part is weapon skins. Every weapon from quake has been redesigned and provided with new animations and sounds. There are no gameplay changes, so classic Quake is in tact.
The second part is a complete remodel of the gore visuals. Gibs have been remodeled to be halfs of enemies, enemies with their cranium removed, holes in their stomach, etc. as well as new generic gibs. Darkplaces users have a little extra, as it includes new blood particles/decals as well.