Haste alteration
I've used Haste/Mass Haste as both a LV 8-14 sorcerer, and as a DM for PCs up through 16. This is in a combat-heavy game. Here are my thoughts:
1) Haste is extremely nice, more so for spellcasters than weapons mavens.
2) Haste does /not/, in my experience, double the effectiveness of my sorcerer- the reason being that my sorcerer can not cast metamagics on 2 spells/round, merely 1. By this level, many of my best spells are lower-level spells with 1 or more empowers. It is still a nice bump, and it appears to double the effectiveness of primary spellcasters who need to prepare spells.
3) Mass Haste typically puts my character in line with the rest of the party for direct damage. Without it, my character is weaker in terms of direct damage. Of course, like many spellcasters, my sorcerer's power is usually also expressed in other "buff" spells for the entire party.
4) By a reasonable level (8+) all NPC spellcasters have one chosen or prepared who can do so. This is even more true for mass haste, when it's avalible. Unlike several other posters on this list, I think that the best counter for Haste is Haste, not Slow, nor Dispel Magic. Unless the opponents are particuarly nasty, Dispel Magic (or its varients) does not have a good chance of success. My reading of the rules suggests that Slow counters 1 casting of Haste, not multiple.
5) Many NPCs without access to such a spellcaster will use wands, potions, or items to get the same effect.
As a player, I've looked at the spell, and asked myself "Would I still cast it just as often if there were no AC bonus?"
The answer is "Yes".
So my thought is that the easy way to reduce its power is by removing the AC bonus. It does weaken the spell slightly, without altering its main effect. Is it still overpowered as a LV 3 (LV 6 for mass) spell? Probablly. But, on the other hand, I feel that nerfing it would weaken spellcasters quite a bit, and I don't feel like doing the additional work I would want to do to address the issue of removing it or replacing it with a substantially weakened version. I'm more interested in playing with other pieces of rules.
Frankly, my opinion is that Heal/Harm/Mass Heal is a larger problem than Haste/Mass Haste.
I've never played with Time Stop, but frankly, as a DM, that spell looks kinda scary - on either side.