The worst abuse I can come up with is a fighter/ranger with doublesword or urgosh. You're essentially stacking fighter striker damage from tempest with ranger striker damage from hunter's quarry, using twin strike. At higher levels with all the static bonuses you can get, damage output will go through the roof, especially since you can also pick up a multiclass feat such as Avenger to become super accurate for 2 rounds per encounter with your zounds of attacks.
Aside from that, it seems balanced, leaning toward the weak side for most combinations (which I prefer over leaning toward the strong side).
There are some interactions that are sort of unclear to me. It looks like a human hybrid something/warlock could start with one something at-will and two warlock at-wills, and then pick up a pact with the hybrid feat for eldritch blast and the pact at-will, giving him 4 warlock at-wills and 1 something at-will, for a total of 5 at-wills. Not sure if this is intended or not.
The wizard hybrid made me chuckle. They get cantrips, yippee. Oh and yeah, that's all they get. Oh wait I forgot, they also eat all of another hybrid class's armor proficiencies for good measure.