The phrase "Linear Fighters, Quadratic Wizards" gets bandied about a lot, but I've found I don't have a good way to explain it to newer players.
The tier system post has some examples of how wizards are better than fighters in specific situations, but I don't find the examples very satisfactory: the wizards in the examples seem to mostly rely on cheesy abuses that wouldn't happen in an actual game. For example the post says a wizard can kill a dragon using shivering touch from Frostburn, or using mindrape and love's pain from the Book of Vile Darkness, but many games won't allow those books.
In an actual play scenario, with no access to any expansion books, and assuming a group of characters that aren't grossly evil: what sorts of trends make wizards (or, more generally, full spellcasters) more powerful than non-spellcasting classes? At what character level does this start to happen, and what spells available at that level are responsible for the change?
I'm interested in responses pertaining to both 3.5e and Pathfinder; if there are important differences between the two, I'd be interested in hearing about those as well.