Thaumcraft (version 3 of which has been added) adds a practical aspect of magic to the pack; YOGSCAST Duncan has successfully proven that it works well in conjunction with science, and it adds a handful of desired features for automation to the pack.

In regards to changes to the game as a whole, it adds a handful of new mobs (Wisps, Angry Zombies, and Firebats); the former two in the Overworld, and the latter in the Nether. Wisps especially are annoying, but generally won't spawn unless Thaumcraft is being used a lot in the area.

In regards to worldgen, the provided configuration will NOT add ores or Vis Nodes to already-generated chunks (but a simple change will permit it).

In regard to receipes, I believe that none of them are visible to NEI; this is intentional on the part of the mod maker, and unable to be configured. However, the receipes are availible from an in-game guide item after proper research (a mechanic of Thaumcraft) is preformed.