To me, the first thing about 4th edition that really stood out and screamed, “Hey! I’m different!” was a rather small thing. Maybe a trivial thing, when you hold it up against the system as a whole. But it was a thing that had been consistent across all previous editions, and even (with small variations) in the old non-Advanced, Basic-Expert-Companion-Master Set D&D. And now it’s no longer there.
That thing is this: magic missile never misses.
Magic missile is the mainstay of many a first-level wizard. It’s the last resort or the ace up the sleeve of many a high-level wizard. It doesn’t do heaps of damage, but it does a fair amount, and most importantly: it always does it. No roll to hit. No saving throw. As long as you can see your target, and they’re in range, you cast it and you hit. It’s reliable. If you cast it at the darkness, then the darkness takes damage.
Or it was.
In 4th edition, magic missile is a ranged basic attack. The wizard rolls to hit. He rolls not against armor class, but against a different passive defense — Reflex Defense — which has, along with its counterparts, largely replaced saving throws. If he rolls well enough, he hits and does damage. It’s a little more damage than a first-level wizard used to get out of magic missile, but less than a high-level one would. And if he doesn’t roll well enough, magic missile misses.
Granted, it’s now an at-will power of the wizard class. The wizard can throw one every round if he wants to, long after even a 3rd-edition sorcerer would have run out of juice. An auto-hit would make the power just a little bit unbalanced.
But still… magic missile can miss. Something that had endured for pretty much forever, as D&D history goes, has changed. It’s a small thing, but it’s also big.