One thing I'm doing in my polishing is changing the lighting. Here is the prefab of Riverbridge before went to work, the time of day is set to moonset.
And here it is after, still at moonset. Building windows with lights in them actually light. The lighting actually looks like it's nighttime. The lightset is based on the "exquisite" set on the Vault, but darker during the night.
The other main thing that get the once over during the polishing pass are conversations. I'm going through the conversations to make sure placeholders are filled in and journal updating looks right (and making the journals multiplayer friendly). Companion interjection and greater pc dialog choice also happen during the polishing phase.
Areas also get some extra work if they need it, for instance the prefab that is serving as Murann was much too "clean" for a city under control of a goblinoid army. It got extensively scrubbed with dirt and irregularity. Now it looks more natural and more like an occupied city.
I am also checking area names, music (most areas have none), making sure areas connect to the overland map, things of that sort. It's an effort to giving everything a "final" look and bring the areas into a state where only bugfixing is needed. Ideallly, after polishing no one will look at an area and say "hey, it really seems like this area is missing an oxcart".