I'll post more when I'm in a computer rather than a phone, but there are a couple of observations.
1. Allowing spells outside Phb accentuates brokenness'. Far too many poorly designed uberspells.
Fortunately my PCs stuck to the PH1.
2. I'm seeing a lot of responses here that seem to assume the wizard is only in 1-3 encounters per day.
3 encounters is pretty standard. It's hard to put more into a day unless the PCs are dungeon crawling, and even then they could leave the dungeon when they want to rest. I'm not sure when Broken Secret Hut becomes available; I never learned that spell as a mage.
And never gets caught by surprise.
I'll tell you what happened to my "invincible" wizard character when he got caught by surprise. It hurt.
He was a 9th or 10th level halfling wizard, probably 10th, and had taken Skill Focus (Concentration) and Silent Spell. (Other feats too, of course, but they weren't relevant in this encounter.) This was Eberron, and we were doing the Xen'drik adventure.* I forget what it's called. Before the attack, his only defensive spell on him was False Life, because ... well, I wasn't a gnome. The only defensive spell he could cast this battle was Silent Dimension Door.
We were on a small boat on a jungle river. The river was really narrow, more like a stream. Narrow enough that the local wildlife (which were riled up by some vague magic effect) could jump on the boat! Well, if they were a dire tiger.
Both tigers jumped on the boat, and one clawed me twice but missed on the bite. Being a small creature, it could easily grapple me if it bit me, and could then jump into the jungle again with me. I'd be far from friends! Confident in my Concentration score, I started to declare that I would cast Silent Dimension Door... except where would I go? By myself into the jungle?
I had to take two full-round attacks from the tiger (it missed with both bites, good thing it didn't have Multiattack) before I had to be rescued. The reason I wasn't rescued in one round? Because another PC was in dire straights too!
I don't know how I survived both rounds, but I vaguely recall either someone cast a healing spell at me, or I pulled off a defense spell -- it couldn't have been Mirror Image or Greater Invisibility, as dire tigers have scent, and it was a core spell, and not an Eberron one since I didn't use those.
Said wizard got into more trouble when, in the dungeon itself, enemies intelligently "flanked" the party (not the +2 to hit maneuver, but attacking from both sides) and I didn't get the customary one round of buffing while hiding behind the fighter before dishing out the offensive magic on the grounds that the fighter was engaged fighting someone else. There was a clear path to me which two opponents could take.
So I just one-shot-stopped one of the attackers with a save-or-suffer spell (I think it was Baleful Polymorph or Hold Monster) while another PC (I think a cleric or paladin) tangled with the other back-attacker. Tactics turned out to be more important than having decent Hide skills, it seems.
It would have gone very badly if the attackers had "artillery". Having said that, unless they all deliberately focused fire on the wizard, he would have lived, gotten healed, and gotten off a good defense spell like Mirror Image. (A single "artillery" couldn't have dropped me, and with my Concentration check, if one had readied an action to interrupt my spellcasting, I would have taken the damage and cast a good spell anyway.)
I even deliberately nerfed my own mage sometimes, although by nerf I meant "not metagamed". For instance, being attacked by some creatures that I (as a player) knew had tremorsense, I didn't make any skill checks and cast Mirror Image anyway, since that's what he would normally do. The monsters were melee attackers though, which meant the presence of the other PCs helped a lot. While there were close calls, I didn't get ganked.
*The DM had upped levels for the whole adventure, and had probably rewritten much of it too.
And never has to wrestle with the opportunity cost of casting spell x vs y in combat.
Proper spell selection.

Maybe you could give some examples. I was pretty good at picking a selection of spells for all situations though.
For instance, running into a golem in said dungeon. Good thing I had prepped Glitterdust. Twice. (This was standard; I had no idea a golem would be there.) One was heightened too, since that spell was probably a little too good for 2nd-level. My wizard's use of Protection from Elements (acid, IIRC) turned out to be more important than his ability to screw over blinded golems
FWIW I think the wizard in 3e was always lower on the power curve than Druid, Cleric and Psion.
On this I agree.
Druids and clerics are just tougher. Catch one by surprise and they tend to have higher AC, hit points and good Fort saves. However, their array of defensive spells were weaker (because they didn't really need them). Psions are complicated, but they generally had more "offense" in 3.5. Their buffing spells were more like those of clerics, which meant they weren't that great for d4 hp/level characters. I don't want to touch 3.0 psions with a 10-foot pole; there were so many horrid issues there.
To some extent, wizards got "overpowered" defense spells
because of their low hit points/AC. You'd never see a cleric with Mirror Image... or at least I hope not! On the other hand, the pendulum swung too far with spells like Greater Invisibility, IMO. Now the wizard enjoys a 50% miss chance (good, but can be dealt with without magic) and can't be seen or targeted (bad, no one had high enough Listen checks, needs magic to solve).