I love questions.

1) Haste? Ghost, do you mean?

2) Clarity? As I've said, you're a support character. Unless you're the only hero on your team that needs mana (which is possible, if Akali, Mundo, Garen and Vladimir are your teammates) you can use clarity to keep your entire team's mana up, while the other team will struggle to keep up with your relentless attacks and casting in a 5vs5 battle.

You can't go into a battle empty and neither should anyone on your team. Your job to make sure everyone's at 100%.

3) If I had 100 gold every time some noob exhaust/ignite + exhaust/ignite team waited for me in a bush and I rushed in...

The bonus HP from the Quintessences makes sure you GET AWAY early game. If they pwn you early on, you're basically screwed if you're not good enough to get back into the fray. So, I give the option of getting more HP so you don't get pwned. Plus, Zilean lvl 1 with ~800 HP? The other team would have a death wish if they tried to pwn you early on, having only around 600 HP.

Also, extra 10 Magic Pen. works wonders early on, because people get Mercury's Treads right away and unless you can go back, you're basically damage-less early game. Remember, the more magic pen. you have, the more damage you deal.

Cooldown Reduction? Obvious.

well this guide is kinda eh... a cpl of questions...
Why no haste? and why do u have clarity? and the runes are kinda eh... +0 for now ;/

@TheNewDeal: I personally love to use exhaust, but not with characters that don't need it. What can you screw up with Zilean if you take exhaust? Well, as you've said, if you don't remember to cast your slow first and the exhaust a few seconds later, you will have wasted the 'double' slow. If you cast them at the same time, it won't be that different from a normal slow.

Decent guide for Zilean, with good mana regen and AP. I'm not sure that Exhaust + Time Warp stacks the "double slow" tho...