Lore should justify gameplay, rather than gameplay justifying lore.
In other words, if my MMO setting does not use magic, but a level designer creates something pretty awesome that requires a hint of magic (floating platforms), you don’t scrap the level because the lore says no magic. You add lore explaining why the new place is special and has floating platforms.
At the same time, consistent lore can help you keep things in check and continue with a good thing. WoW started off as high fantasy, but the definition of high fantasy is seriously stretched when you include space goats and Paris Hilton. Which is not to say such things are outright bad (ok Paris Hilton has pretty much zero redeeming qualities), but we have seen plenty of people annoyed with the direction WoW lore has taken. I get that the Alliance needed an ‘ugly’ race, but there are plenty of high fantasy choices that would have fit better than space goats.
Another good example is the Prophecy of Pendor mod I’m (always) playing. The setting in Mount and Blade is great because it does not use magic, giving combat a very real feel. With that said, PoP adds demonic legions and other fantasy-ish characters. Still no magic in combat, but not just a pure medieval setting. You still use a sword to hack down the bad guy, but in PoP the bad guy might have an unrealistic amount of hit points, along with looking a little inhuman.
It’s a fine line, and every game needs to find it and learn to balance on it rather than falling over, yet when push comes to shove, gameplay should always win, even if it is immersion-breaking.