Where the Slayer and the Fighter compare:
The Fighter AoO abilities (Combat Superiority and Combat Challenge features) would be offset by the Slayer's improved saving throws, resistance to enemy AoO, and increased damage on AoO.
The upgrades to the Slayer's Power Strike also have to be weighed in as a little better than equivalent level Fighter encounter powers. Consider that any upgrade to the Slayer Power Strike immediately upgrades 100% of his encounters attacks and applies to AoO and granted attacks from other classes.
All that taken into account, up to that point everything is equal IMO.
Where the Slayer and the Fighter contrast:
Daily powers of the Fighter might be too much for the Slayer to overcome. Fighters gain dailies that guarantee damage to adjacent enemies as an effect for the duration for the entire encounter. Automatic damage like this makes the Fighter overlap with and outshine the Slayer.
How to favor the Slayer:
1) The Slayer does not use dailies. IF you play a game where milestones are reached frequently, then the Slayer will hold his own as the Fighter cannot recharge his Dailies between big fights.
2) IF a Slayer plays alongside a class that grants MBA, the Slayer will shine. In fact, this might be the best way to use several Warlords in a fight. Mixing up Warlords and Slayers would generate excellent damage output.
3) Magic items that grant bonuses on MBA are great for the Slayer and tend to come at lower levels than Items with prized daily abilities.
4) Slayers do not rely on strategy. Strategy can be awesome, but sometimes it does not work out as planned and does not make use of the Fighters class abilities to the same extent.
Several things favor the Fighter:
1) Daily powers will dominate if the party wants to have extended rests between fights. This is common because all the other classes in the game pretty much rely on their dailies. If a Fighter can use two dailies in a fight, then the Slayer will fall behind.
2) IF a Fighter gains the ability to use an At-Will attack in place of an MBA, then it chips away at any advantage that the Slayer would have.
3) Fighter AoO already have advantages such as marking, stopping movement and having Wisdom added to attack rolls. Maybe not as good as outright improved damage on AoO (as the Slayer has), but definitely useful.
4) IF monsters avoid AoO entirely, then the Slayer will be very limited in damage output. The Fighter can generate an extra attack via his Combat Challenge more easily than relying on the Monsters to make mistakes.
5) FIghters can better move monsters around the battlefield, either over ledges or into damage zones. The Slayer does have a limited ability to do this, so it is not a complete win for the Fighter.
The Slayer is a viable class as long as the party and the DM style is built to make use of him. I don't think that the Slayer is ever better than the PHB fighter, but can be the Fighter's equal under the right circumstances.
nnms is right. The best Slayer is a charging slayer. A Slayer should be impetuous and in the middle of a melee. He is resistant to AoO and effects that slow him down and he wants enemies moving around him to generate his own MBAs.
The Slayer should not want to rest up. I wouldn't choose any Daily Utility powers for this reason. You want to be at full offense for every encounter of the day. Emphasize the milestones and ignore the urge to recharge daily powers and healing surges.
The Slayer wants to get there first and just hit something. Less thinking, more stabbing.
Think Leroy Jenkins.