Netherworld really shook up the Shadowfist playing field, which actually wasn't that hard to do at the time — many groups had degenerated into cheese fests (event-heavy Ascended and Architect decks dominating) or waxy character buildup prior to one massive attack for the win. Netherworld changed that primarily by introducing alternate power generation cards, an antidote for the "turning" sites (Kinoshita House, etc.), plus a few ultra-good hitters that were immune to many of the anti-Character cards.
The alternate power generation was a mixed bag. Pocket Demon gave a much-needed boost to the Lotus and Monarchs, and unfortunately also boosted the Architects (pesky DNA Mage!). It also enabled the single-column deck (usually with a Proving Ground in the back row) to become a truly viable play strategy, allowing the player to burn for victory and still generate lots of power. The Hand got Violet Meditation, which wasn't quite as good as Pocket Demon in most metagames (well, ok, Violet Medication is a Chi card, but who else uses Chi regularly?) The Dragons got Kiii-Yaaaah!, another metagame-dependent card. The Jammers got left out in the cold, no power generation for them.
A number of other cards were introduced to try to help comebacks, like Fortress of Shadow, Garden of Bronze, Shield of Pure Soul and Heat of Battle. Many of these only worked if an opponent was closer to victory than you, which of course led to decks that purposely tried to play from a little bit behind so all these effects could be used.
Whirlpool of Blood introduced a much-needed effect to Stop the Madness(TM) of the overpowered "turn to annoy my opponent" cards from Limited Edition (Fox Pass, Kinoshita House, Turtle Beach, City Park, Proving Ground, etc.). Whirlpool became a de facto standard for deck building, especially in dueling; the only question was how many went in.Thank-you to Stephan Vincent for providing some information for this page.