http://www.worldofmunchkin.com/faq/changelog.html

Starting with the 19th printing of Munchkin in May 2010, we made some changes to rules and cards across the Munchkin line. This page lists those changes for the sets that have already been released or are at print. We recommend bookmarking it and checking back frequently for new sets!

• The starting hand is now four Doors and four Treasures.
• We added a “Contradictions Between Cards and Rules” box, detailing the guideline that cards supersede rules. A few special rules always supersede card effects unless the card explicitly says otherwise.
• We explicitly state that you have a chance to play cards and alter your layout at the start of your turn before Kicking Open the Door. (That was always the intent, but the rules didn't make it clear.)
• Taking a monster's Treasure is no longer part of the Loot The Room phase. This removed ambiguity on several cards.
• The “friendly monster” clause was removed from the Loot The Room text. “Friendly” is never defined in the rules, and almost all monsters that could be considered friendly already say what to do if they do not fight, so this was both confusing and superfluous. The rule now is that if you encounter ANY monster, you cannot Loot The Room.
• Charity now explicitly says that you only give charity after playing (or otherwise getting rid of) all the cards you choose to. Additionally, we clarified that it depends on hand size only – the first version of the rules was not explicit about this.
• Combat now draws a clear distinction between Level and combat strength. We tried to remove the terms “base level” and “effective level” from the game, as they sometimes led to confusion.
• The rules for fighting multiple monsters now have their own subhead.
• “Asking for Help” clarifies the distinction between tossing cards into a fight and actually joining the combat, defining the term “helper” explicitly. It also says that the main fighter always draws the Treasure, even if it was the helper's ability that won the fight.
• Moved the “Interfering With Combat” section to a box under Combat.
• Running Away now ends with “Discard the monster(s).”
• Clarified the Death rules. You resurrect as soon as the next player's turn begins. At the start of YOUR next turn, draw four Doors and four Treasures and then take your turn normally.
• Added the rule that dead characters cannot receive cards or level up.
• Under Treasure, added the rule that Go Up a Level cards may be played on any player.
• New box: “Example of Combat, With Numbers and Everything.”
• New box about smartphone level counters.
• Added an explicit clarification that you cannot discard Items “just because.”
• Moved “Trading” under “Items.”
• Selling items now clarifies that you cannot sell less than 1,000 Gold Pieces worth.
• Changed “Treasures – Playing Them” and “Treasures – Using Them” to say “Items” rather than “Treasures.” Added an “Other Treasures” box.
• Added a “Super-Sized Munchkin” box and rewrote the Faster Play Rules.
• New! Steeds and Hirelings (except for Mooks) may no longer have Monster Enhancers played on them for a bonus.

• Super Munchkin now works like Half-Breed. It can be played with one class, giving all its advantages and none of its disadvantages. (Ultra Munchkin and Super Duper Munchkin have changed similarly.)
• Cheat! must be played with an Item card from your hand or on an Item card you already have in play. (This is not a change in intent, just a clarification in wording. Cheat With Both Hands has also been clarified.)
• Wandering Monster now says “add their combat strengths” rather than “add their levels.” Similar changes were made to MANY other cards that said “levels” when they meant combat strength.
• All Monster Enhancers have had “Level of” deleted from their modifier description. Negative Monster Enhancers no longer say “Minimum Level 1,” either.