Is that magic spells and items are so clearly unsystematic and ad hoc that a GM really felt licensed to add anything he could possibly think of. The only unifying principle was that more powerful spells should be higher level and the more powerful items rarer; other than that anything goes. So our early D&D games were full of fabulous spells (often realized as bizarre dungeon effects that messing with this or that statue or altar would invoke), items, creature abilities…we let our imaginations run riot.
My strong impression, though maybe this was just a fault of mine and the people I tended to game with is that later systems tended towards either providing a toolkit to build spells (e.g. Fantasy Hero, Ars Magica, BESM) or a more-or-less exhaustive list of spells that you were expected to keep to (RoleMaster, etc). The idea, laughable in the context of classic D&D, was that some effort had gone into thinking about the system of magic and balancing the effects, their costs to learn or cast, and so forth. If you messed with it, you did so at the peril of throwing things out of balance or introducing a contradictory mechanic. If the toolkit didn’t provide an appropriate base effect, or the cost of the modifiers needed to make it useful were completely out-of-whack (because those same modifiers applied to, say, a spell that did damage would make it devastating)…well, you were free to add or adjust it, but there was a definite impression that you were messing with something finely tuned that might not work as well or at all once you got done with hot-rodding it.
Eventually, there came systems where everything was defined more-or-less by the same mechanic (e.g. something like the PDQ System or Dogs in the Vineyard) , whether it was casting a spell or catching a fish, so questions of balance pretty much went out the window. So did a lot of the sense that there was something special about magic…it’s a little hard to explain, because I’m not sure I completely understand my objection myself, but if a system is too abstract and rules-light I start to lose the sense that there’s anything about magic that’s any more unusual or mysterious than fixing an engine, because the player goes through the exact same steps with the same mechanic whether he’s casting a spell to summon a whirlwind and transport himself a thousand leagues or change the spark-plugs on his Chevy Nova. There might be setting information that makes one possible and the other inconceivable, or modifiers applied, but there’s something that’s kind of flat and abstract about it.
The feeling I got from classic D&D was that half the fun for the DM was to make up wilder and wackier spells and items, either for the players to use or to be used against them. Dave Hargrave’s Arduin Grimoire was a notable example of just how wacky it could get, but all the DMs I knew did the same kind of thing, albeit perhaps on a smaller scale. I actually had a long-running campaign in Arduin, that my brother still thinks is possibly the best–or at least most memorable and atmospheric–one I ran.
Nowadays I’m much more likely to feel justified just house-ruling the heck out of everything to get it to where I want it; but nowadays, I’m much less likely to even be GMing a published system. One of the things I admire about Savage Worlds is that while it basically is a hybrid of the toolkit and grimoire approaches to magic systems, the advice to GMs (at least in the Fantasy World Builder’s Toolkit) is much more reminiscent of the anything goes feel of D&D. So while spell that shoots a bolt that damages a target is the same whether it’s a blast of fire, a magic arrow, a summoned swarm of bees, there’s no attempt at making an accounting system for balancing the duration, range, area of effect, etc of spells against each other or some standard point cost. If you want to add a new spell, you’re advised to either just change the “trappings” of an existing spell and add new minor mechanical effects as appropriate (e.g. a bolt of fire might set things on fire in addition to the direct damage, a bolt of ice might slow them or cause a slippery patch on the floor), translate a spell from another game, or just create it from whole cloth. You could certainly use the spell lists from SW and no more, but like old school D&D it cries out for and gives license to expansion in whatever direction your imagination takes you.
I think it can be summed up as: when the system makes no attempt to balance spells against anything except a difficulty rating or fit them into any kind of taxonomy or metaphysics, it’s clear you can just toss in anything you like. When the system obviously has attempted more than just a list of really cool things you can do with magic and has put some thought and care into it, then as a GM you feel like you ought to be doing the same. And sometimes I miss just saying, Ooh, wouldn’t it be cool if there was a spell that did X? Let me write that down….