YMMV / Ars Magica

• Audience-Alienating Era: The Third Edition was published by White Wolf at the height of The '90s, and partially integrated into the Back Story of Mage: The Ascension. Accordingly, it shares all the hallmarks of their "Gothic Punk" approach of the time; Infernal plots around every corner, attempts to deconstruct the medieval society and its views, the flawed idea that rationalism and magic were incompatible, darkening or questioning the character of every historical figure on the stage, and padded wordcounts with little actual content. This was poorly received by the player base, and when Atlas Games acquired the license, they broke the game away from the World of Darkness and reversed course at full speed, making Ars Magica once more its own beast.
• More Popular Spin-Off: The Old World of Darkness. It was created by the same people and incorporates many of the concepts of Ars Magica, and the WoD and 3rd Edition AM were linked together. The two games have gone their separate ways, however, and while they share concepts like the Tremere and the Order of Hermes (and the owners of the two lines don't fight over ownership of either concept), they're no longer linked.
• Scrappy Mechanic: 3rd Edition attempted to include a "Realm of Reason" (most likely to bring Mythic Europe in line with Mage: The Ascension's backstory), which was based on skepticism and scholarship, and which gave off an aura which mitigated the power of other Realms. This proved highly unpopular, not only because it was paradoxical and inconsistent (for example, Reason's presence in libraries meant that casting spells there was penalised, despite magic always having been portrayed as a scholarly and academic pursuit), but also because applying reason to Mythic Europe should confirm that angels, demons, fairies and magic actually exist; denying them comes off more as delusion rather than reason. It’s made even worse when you consider that Hermetic magic embraces the type of idea-evolution that goes into the practice of science; several Bonisagus traditions are founded on the very concept of adding new ideas to Bonisagus's theory. 4th Edition did away with "Reason", of course.
• That One Rule: Among all the technical rules of Ars Magica, none has caused as much hair-pulling as Magic Resistance. As written, if an item with a magical effect on it strikes a magus or creature with Magic Resistance, and the spell fails to penetrate it, then the item itself is stopped and cannot cause any harm to the magus; this is known as the Pink Dot rule, after the standard thought experiment (where a magus uses a spell to cause a tiny pink dot to appear on an attacker's weapon, thus making the weapon magical). Any other coherent alternative proposed has led to worse problems where one's Magic Resistance can be exploited against them. Fortunately, Rule Zero applies in actual play, and any attempt to intentionally abuse the Pink Dot rule will result in being hit upside the head with the corebook.

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