There were a number of new concepts introduced into the AD&D magic system, which I now wonder how I lived without.
Wild Mages are a lot of fun, quite powerful, and almost as deadly to the party as to enemies (thus balancing them out with other mages).
New ways for priests to cast powerful spells, make them seem more like holy warriors, and less like walking bandages.
Metamagic: spells that affect other spells, is also an interesting and much needed addition to the spell lists.
If you like spellcasters, get this book. If you don't like spellcasters, get this book. It may change your mind.