I chose the limitation on spell selection, reliability, and resource cost.
Selection: choosing which spells to prepare for an adventure should be an important decision. This important part of magical balance should not be rendered meaningless through the availability of cheap,and plentiful wands and scrolls of whatever spell is desired. Choosing to prepare the knock spell or not becomes a no-brainer if you can just pick up an easy wand loaded with 50 of them.
Reliability: casting magic in combat should be somewhat risky. This important balance factor was trivialized in 3E and thus caster dominance began to grow.
Likewise, not all magical effects need to work flawlessly. Teleport should be very risky if traveling to a location that has not been visited personally.
Resource Cost: the old maxim of the easily obtainable being held in contempt applies here. Magic should always have a cost, however small.
Truly unlimited at-will magic, should be reserved for deities and other beings of such power. Anything that can be done literally all day long without cost or effort will quickly become mundane.
In my own B/X expansion material, magic users have access to lesser powers and cantrip style magic. The mage has 1 power point per point of intelligence to spend per day on these powers. The powers have a variable cost (1 to 4 or so) depending on what they do.
The magic user can thus, do magical things beyond the 1 spell per day to start but no magical power however slight, is truly unlimited.
I agree with some of this for example the issue with scrolls, wands being way to easy to make.
I always liked how 3.0 did scry not only did you have to have the spell and focus but you had to have skill ranks in it which controlled how well you did. This was a nice way to handle it being abused. If someone wants to pour ranks in then it should be easy for them to but by pouring ranks into it they are having to take them from somewhere else.
I think teleport without error should just be done away with. There should always be a chance of ending up somewhere else.
I don't want to go back to Ad&D where any damage you take means you lose a spell. Personally I have found that 3E way works fairly well. If you get hit it is not a given that you are going to be able to roll high enough to not lose the spell.