If your going to be a pure mage then Alteration is a good start, its the best way to keep yourself protected.
Destruction is good up until a certain point and then it seems to be mediocre -although i found that Ice Storm is the best destruction spell to use-
Conjuration is pretty useful, especially when you get the high end perks.
Illusion can be a very useful tree when mixed with other -such as conjuration-, using the spells to make the enemies run or boost your own summons & followers, means you'll be able to handle most fights.
Restoration is useful for healing -of course- , blocking magic attack -wards very good when mixed with atronach and magic resistance spells- , and dealing with undead.
Enchanting is of course good for enchanting things, meaning you can be left with very good magicka and magicka regeneration rates. Plus resistances to different types of damage.
i think thats all.