So, I'm thinking. We have colleges of magic that provide a handy way to "channel" magic users onto a particular path to power. We have clerical domains which define a cleric's deific proclivities. Didn't spell lists do that back in the misty ages of yore? At least sort of.
Look at the Illusionist. I'm too lazy to go rooting about for more examples. If you read this blog, you already know. You may not agree with my assumptions, but you know that there are a multitude of unique spell lists fine-tuned for specific classes. I believe that spell lists were an early effort to "univeralize" at least one aspect of AD&D. Second edition took the colleges of magic established in 1st edition and used them as a tool for defining "subclasses" of magic users. It extended the concept to clerics with the notion of Domains.
Now, I want to make clear: I think it was a good direction. Unfortunately, the more one-size-fits-all a thing gets, the more likely it is to not fit quite right. There may be a certain group of spells that a class should be able to cast, but they belong in a category with spells that have no place in the class concept. Perhaps you want a class to have limited spell casting, not just categorically, but in absolute variety, as well. That is where individualized spell lists come in.
With individualized spell lists running hither and yon throughout a campaign, the question becomes "Can my magic user learn/use spells from a subclass' spell list?" My personal answer to that is "Sort of". My ruling would be that a character could learn from another's spell list if it is of a compatible type of casting, clerical or arcane. If that condition is met, then the character has to research the spell, essentially converting the spell into a format they can utilize. The character would receive a bonus to his research attempt should he have access to a spell book containing the spell, or expert instruction from a caster who knows the spell.
I'm thinking more and more about implementing limited spell lists for certain classes, or ones less limited but still unique, a la the illusionist. I'm not dead set against rangers or paladins having some spell-like abilities, I just feel more comfortable with the idea that they have a much narrower selection. In my mind it makes it easier to view them more as magical abilities, rather than spells. I hate to paint it this way, but I will: they are like Daily Powers. There, I said it.