The data.000 file basically defines your character (stats, skills, experience points, class, gender, etc.) and keeps track of all NPC locations and status, in addition to storing your own location, and some quest information.
If you replace the data.000 file with a previous version, you would loose the experience, stats and skills gained since then. I think most quests would probably be ok (remain solved if you solved them, or be solvable if you have not) but you would loose the experience points and any skill rewards for those completed since the previous save. You would keep all equipment and items in your inventory, in containers or on the floor. Monsters you killed since the previous save would be restored, treasure would stay looted.
