Here's a couple of less thought of immersion breakers that people were sort of discussing on Tobold's blog today. And yes there is a trade-off here for each one.
1. Scalable Bosses/Dungeons/Loot: What, this is an immersion breaker? Yes. It reminds you that its just a game, and it doubly reminds you just how fake those pixels are, and it triply reminds you that you could just solo it or just play with your closest buddies. Personally I love this mechanic when done right but I've been thinking about it and the dungeon .. each dungeon that uses this, loses some "authenticity". It ceases to be a single dungeon and becomes something stricken with multiple personality disorder or something.
2. Drastically changing the leveling exp curve: What could be wrong with changing the exp curve to be more forgiving? Well if its a super evil one and you make it more gentle, maybe nothing. However Lotro's was already pretty well balanced with how long it should take you tog et through the content and they nerfed it, thus breaking crafting by making you have to grind it instead of naturally collect ingredients along the way. The amount of time spent in Tier 1 areas is blindlingly short, too short to advance to the next tier. (for example)
3. (Cuz I like the number 3) Stuff that happens and doesn't make sense. Bosses behaving cowardly for no reason? Check, see Lotro's rock boss in the three man waterworks. Treasure hordes you can't loot? Check see grand stair. Bottomless pits in a world where the lore doesn't allow it? Check. Any others I missed? :)