1) Try to solidify your PC's concept before deciding on things like feats. Sometimes, your concept will completely determine your feat selections.
2) Personally, of the feats, I like Energy Substitution (for flexibility), Still Spell (for armored mage builds), and Reach Spell (for flexibility & the surprise factor).
3) If it fits your concept, specialize. The extra spell may just save your party. And while I'm not a big fan of the PHB2, I must confess I like the alternative class feature of Focused Specialist- +2 more specialist school spells (in addition to the ones you already get) in exchange for another barred school. What wizard doesn't want +3 spells per level? (And for Diviners, this is a steal!)
4) Look into other alternative class features- you'll find them in Dragon Magazine, Unearthed Arcana, Complete Arcane and other sources. Dragon Magazine recently published one called Eidetic Spellcaster- you no longer need spellbooks to prepare spells- instead of Scribe Scroll and a Familiar. I really like that one, partly because most of my current DMs' hostility towards pets- familiars, animal companions, special mounts, etc. tend to die pretty quickly. At least with the Eidetic Spellcaster alternative feature, my Wizards have something that is always useful.
It's pretty important, since everyone packs magic missile.
Not quite everyone- in 30 years of gaming, I've never run a mage who even has it in his spellbook.