Picture a setting where the average tavern is using Longsword+1s as spits for flanks of beef, where every household features a Decanter of Endless Water, where Beads of Force are scattered amidst the pebbles (okay, slightly hyperbolic here, but not by much) and where you are likely to uncover half-consumed Potions of Healing in a residence's refuse; that abundance of magical items sets a specific tone. While, yes, the availability of these treasures does carry some long term mechanical (i.e., game-related) consequences for a D&D campaign (specifically, it makes creatures impervious to mundane weaponry less threatening and increases overall survivability), it also diminishes the wonder and appreciation associated with discovering what is ordinarily an in-setting rarity* and real-world impossibility. Now, picture an adventure where the PCs journey into the wilderness and chance upon deep forest ruins; after cheating death twice over in what remains of the central tower, they reach the pinnacle only to find a skeleton clad in battered field plate while still clutching a blade curiously untouched by rust. Big difference.
Also, the way in which the PCs and NPCs speak and think has zip zero to do with system mechanics or winning/losing. When the bartender says "Hey, dude, how's it hanging?", it creates a different atmosphere than "Traveler, what burdens do you bring from the road?"...even if they both amount to the same sentiment. Considerations that many contemplate for D&D also apply to Pathfinder, Warhammer Fantasy, HERO Fantasy, GURPS Fantasy and even collaborative storytelling completely removed from any sort of ruleset.
* "High magic" settings exist and can be fun, but that is purely a matter of taste; what one considers "mundane" or "fantastic" in that context is likewise subjective and the point of this extended conversation.