For example, let me compare one Codex spell (Philosopher’s Cottage) with a very similar spell in L&D, it can give you an idea of the differences. You are probably familiar with the book Ars Notoria (or Ars Nova), one of the books contained in the Lemegeton. It teaches a system of mnemonics and visualizations, inspired possibly by Memory Palaces, with prayers to spirits etc… to develop eidetic memory and so on. In L&D, if a magician gains a copy of the book, he may study it for a lot o months (can’te remember, 12 or so), after which he can attempt a ritual, which, if successful, will transport him to an extra-planar room where time passes more slowly than on the real world (1:2) and where healing also is at double rate. This is similar to the Cottage, but less powerful in the sense that the time available in the extra-planar room is less than in the Cottage, and also, the magician must bring his own stuff to the room (with lots of limitations). But… from then on, the magician gains the ability to cast the spell (no longer a long casting one, just 10 minutes casting) on any door, and when he goes through, he enters the room; he can come out through ANY door that he has ever passed through (providing a very good means of traveling). This is quite powerful (I’d probably reduce it to “come out through any door he has ever cast the spell on”, so the mage has to slowly buid a list of doors through which he can pass). This “spell” is fully learned, there are no slots or memorization to do. If the mage fails at his DC roll, he can retry some time later, etc…
More, once the mage has learned this, he can continue studying the book, and after a few more months, he can try to learn a new method: he can cast a trap spell on any door he wishes, and seal it with a password. Anyone who goes through the door without saying it will think they are entering the room beyond it, but they suddenly trapped in an extra planar “oubliette”, a cell from which they cannot exit unless they have means of traveling through planes etc… This is quite a powerful ability.
In L&D almost all magic is like this: the mage learns a specific magic skill which he can then use at will (there are of course other limitations, risks, material components etc…), some skills can be learned as he goes up in level, other skills can be gained from studying books and so on.
The core ability of most mages is to summon a demon, and then compel him to perform services. Demons are then listed with the things they might be able to do for mage, stuff like “can reveal a secret from any one person, can discover a hidden object provided the mage can name it specifically, can cause a person to fall sick, can raise a storm at sea until the next sunrise” etc… But there are no “spells” per se as in the D&D game (or the Codex). It’s an interesting system, and one I might combine with the more classical D&D/Codex one.