One With Everything is a Monk passive skill unlocked at Level 45, which improves elemental resistance.
No synergies with any active skills or other Monk passives are known.
Passive skills have had a long and storied history. When Diablo III debuted at the WWI 2008, skill trees included passive skills, much like in Diablo II. In 2010, it was announced that passive skills had been renamed traits and separated out from active skills. At the July 2011 Press Event, Blizzard announced that passive skills were once again in the game, replacing the traits system.
One With Everything was not seen until it showed up in the Diablo III beta as a level 28 passive. It was later dropped to level 24, and when passives were reorganized for Beta Patch 14, One With Everything was moved up to level 45.
In early June 2012, Blizzard commented on how this passive became much more powerful post release, thanks to the gear available in the Auction House.
Three weeks after launch player’s gear is much higher than what we were expecting. When I killed the Butcher on Inferno for the first time I was using a weapon with 492 DPS. There are also certain passives which are much more powerful than they were during internal development. One With Everything, for example, was basically never used internally because we didn’t have an auction House. With the auction house, it feels like a mandatory passive. In retrospect we should have seen it coming. In the game’s current state though, it’s a powerful Monk ability that gives Monks a big survivability boost and has some interesting (some would argue fun, others would argue negative) effects on gearing.
The theory with this seems to be that while using self-found items, players will tend to have resistances that are roughly equal across the board, and thus have no reason to use this passive. However with the Auction House providing much greater access to specialized equipment, players are easily able to stack a single resistance to a very high level, and then use this passive to gain that benefit across the board.