This is a long, long way from my experience.
In AD&D, for instance, it would make a huge difference to apply damage from a fireball to enemies only, or to all creatures in the burst.
4e doesn't stand out at all in this respect.
The major distinction is that in AD&D, the rules adjudicated the fiction -- a big ball of fire clearly does damage to all creatures in the area it occupies, friend, foe, or innocent bystander, because it's a big ball of fire. Unless it states otherwise, it probably damages objects in that area, too, because that's what a big ball of fire would do. What it is defines what rules you use, so it's easy to adjudicate as a DM.
"Oh, that line says it only targets enemies? That's silly, it's a big ball of fire. Your friends need to make a save, too. Don't like it? Don't use it."
The system's disregard for precise balance meant that this was pretty much clearly within the bounds of the DM to call -- DMs were encouraged to make similar rulings. If it screwed the player over, that was "fine" from a system standpoint.
Then errata comes out and corrects the error and it conforms with what the DM says. Or the errata never comes out and that particular player just doesn't use that particular spell and the game doesn't sweat it much even though the DM is changing the rules of the game on the fly to suit her view of the world.
In 4e, however, an effect that may generate a big ball of fire plays a specific role within the structure of the game itself, and ultimately the description must conform to the bounds that the rules of the effect set. That is, the fiction should flow from the rules. So an effect that generates a burst that deals fire damage to all creatures is a different effect in the fiction from an effect that generates a burst that deals fire damage to only enemies. One implies a level of control and precision that the other lacks. In addition, whether or not it affects only enemies or allies as well plays a key part in comparing it to other abilities of the same level, and, indeed, may be one of the power's defining traits in that comparison ("Oh! This doesn't hurt friendlies!").
Now, a DM needs to tread more carefully. If the rules describe a power that only targets enemies, and the player describes it as a ball of fire, the DM shouldn't just rule that balls of fire of course burn everything in their area, because that might be an unfair stipulation present on a carefully balanced power that reduces its utility in comparison to other powers of that level. Just nerfing a power like that might clearly go against the intent of the design here -- DMs can't just go around changing targeting lines like it doesn't matter.
"Oh, that line says it only damage enemies? Weird. Well, could you describe it in some other way?"
The system's more cautious balance means that a rule not in the spirit of the power's intent reduces the character's utility and effectiveness. The targeting line isn't just a description of an effect, it's part of the player's
choice in gaining that power.
Then the errata comes out and corrects the error, and, well, it looks like it was a ball of fire all along, the DM was giving out traits that were beyond the intended scope of the power. And maybe it's not been a big deal, so what's she gonna do, nerf a power her player has come to appreciate? Or just eat the fact that she's made a power more potent and shrug and sigh about it? Or maybe that's not the problem as much as it is that the description of this power that the player's been using now has to change. Great, the fiction changed because the rules updated. Or maybe the errata never comes out, and everything's hunky dory.
This isn't just a hypothetical, either. I've seen errata in 4e do this to people. "Oh. That's not how I envisioned this power. Guess I'll change it now." or "What? That big of a nerf? Well, I guess..." or "Sorry, guys, I know we've been playing it like X, but I guess it's like Y now, so lets try to do it right, OK?"
I don't want errata in 5e to have that big of an effect. While I don't want to dismiss balance entirely, I also want to be able to use the AD&D method of, "Well, it's a big ball of fire, here's what it WOULD do," and not have the game balk at that.