Of course, this begs the secondary question: if we make all those bonuses typed (e.g. "you get a +1 class bonus to AC", etc.), would multiple Hybrid Talent be balanced?Since most class bonuses are untyped, stacking multiple hybrid talents would be pretty abusable.
If a fighter|rogue had Tempest Fighter Combat Talent, Rogue Tactics Brutal Scoundrel, and Rogue Weapon Talent, that would yield some pretty crazy accuracy and damage with a pair of daggers.
Since most class bonuses are untyped, stacking multiple hybrid talents would be pretty abusable.
You can take hybrid talent more than once -- by the hybrid paragon path option. This gives you a second hybrid talent, which is quite good.
The hybrid talent feat is a feat that is "too good". The cost you pay for it is, quite honestly, nominal. Examine the scale of the things you get when you take hybrid talent -- they make normal feats look tiny.
You could fit it into a powered-up game. Allow hybrid characters to take 2 hybrid talent feats, and multiclass characters to take 1.