OneChoice tries to help you reduce quest reward choices down to, ideally, one choice.
Blizzard's quest UI marks items your character can't use in red so you know they're unusable. For classes that can use multiple types of armor this leads to a lot of choices, but your Mail wearing Warrior or Plate wearing Death Knight probably won't be interested in Cloth armor. So OneChoice marks all armor (except relics, rings, trinkets, necklaces and cloaks) that doesn't match your character's preferred armor type red as well.
In addition to this OneChoice places a gold MV below the name of the item that vendors for the highest price. In cases where multiple items tie for this honor, the first one found ends up being marked.
A recently new feature is the addition of a iLevel differential display. This compares the iLevel of the item being offered and the one you have equipped. It will display the difference on items that you can use.
This guidance show up on the quest logs as well.
Note: I attempt to dynamically determine the localized string from the various armor types. This should allow it to work without issue for people playing on non-english copies of WoW. This ability however hasn't been tested so maybe not. If it doesn't work the first time it loads logging out and back in may fix it.