Scything assault: Auto-dazing in a burst as an encounter power is broken at any level. The daze effect should only occur on a hit. Other than that, it's fine.
Force of Spirit: I like the ideas behind the power, but I have a hard time seeing how this isn't totally broken. The fighter is spending healing surges--making him worse at taking damage, and therefore worse at helping the party and doing his job--to do sick amounts of damage. No power in the game--except maybe for a full five-hit Blade Cascade or Assassin's Point with a sneak attack--does as much damage as extra damage equal to half your total hitpoints.
I'd follow the Blood Mage here and let the fighter just take damage, rather than spend healing surges. This makes the power riskier but brings out pretty vividly what the fighter is putting into it. I'd also keep things sane by modelling the massive damage just by lots of weapon damage--8[W] would be pretty generous. The nice thing about that is that it would let the character emphasize, if only once a day, just what big weapon dice can add up to. Maybe something like:
Force of Spirit
Colossus Blade Attack 20
Through sheer force of will, you empower your weapon to deal a massive amount of damage with your next attack.
Daily ♦ Martial, Reliable, Weapon
Standard Action -- Melee weapon
Target: One Creature
Attack: Strength vs. AC
Hit: 8[W] + Strength modifier + Constitution modifier damage.
Effect: You take 4d10 points of damage.
DanChops
His words are wise
His face is beard
So really, the only question is: Do we raise the FoS damage to 4[w] or not?
I like the sound of the second one, but seeing the math done makes me think I should make it 2[W] + Str + HS and push 2 and knockdown. I'm not really sure. Grr...Maybe I should just make it your second suggestion and be done with it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.