Which decisions carry over from the first two games?

There are 5 choices from the Witcher 2: Assassins of Kings that carry over...

1. Did you spare/save Aryan La Valette or kill him in Witcher 2's prologue
2. Did you side with Vernon Roche or Iorveth in chapter 2 (aka did you take roche's path or iorveth's path).
3. Iorveth's path only: Did you save Triss at Loc Muinne or lift the spell on Saskia? Roche's path only: Did you save Triss at Loc Muinne or save Anias La valette?
4. Did you warn Sile de Tansarville about the defective megascope and save hr or let her die?
5. Did you kill Letho or let him walk away at the end?

While #3 is really two separate choices you only get to make one or the other based on which path you took in a save file. Importantly you need to load the auto-save the game makes after the confrontation with Letho for that choice to be carried over.

Additionally if Geralt happened to get a tattoo while carousing with the blue stripes and ended up keeping that tattoo he will still have it in Wild Hunt.

Nothing from the original Witcher game carries over.

Witcher 2's save import didn't actually make a lot of difference in your gameplay. From my own experience I simply had better starting equipment (the best swords & armor from witcher 1) which were quickly eclipsed by craftable weapons and armor in the first chapter at flotsam.

Players without a save still can make these choice in the game

At the conclusion of the tutorial/starting area (White Orchard) Geralt is summoned to see someone very important and while he is being prepared for this meeting is asked a series of interview questions if a save was not imported allowing the player to make and incorporate these choices into the game.

Source: Gamespot