And, now to specifically address a few powers, throw in a few suggestions for some of my own, and maybe brainstorm a few magic items:
First off, Weaken Step, the at-will ability. Instead of slowing creatures it hits, perhaps have it just reduce their speed by 2 or 3 until the end of next turn? Slow at will seems a bit harsh.
Enduring Hex: Instead of having it just sit dormant (I suppose it only adds to the damage of other abilities that detonate hexes, or rely upon hexes to deal damage), perhaps you could do something like:
Enduring Hex Necromancer Attack 3
You case a hex that lasts for an entire battle.
Encounter * Shadow, Implement, Hex
Minor Action Ranged 10
Target: A creature
Attack: Constitution vs. Fortitude
Hit: The target takes 1d8 damage. Target takes -1 to all rolls, including saves. This effect ends after 3 saves have been successfully made against the effect.
Note: For added endurance, you could make it so that two of those saves have to occur without a failed save in between... or even all 3.
Dead Man's Cry: You might want to make it so this ability is only recharged if the opponent is standing on a new corpse when you use it. Each time, the cry comes from a different body, and it also limits the amounts of time this can be used in each encounter (unless you happen to have a LOT of dead bodies).
Unstoppable Minion: I love this ability, but you might consider making it so that, to sustain it, you have to spend either a healing surge or 10 hit points. Perhaps you could even add that you can't sustain it if you have no healing surges left (even if you intend to sustain it by taking damage, you can't if you have no surges left).
Devour Alive: Maybe have this heal as if the minion had expended a healing surge, but can't be healed by any other healing effect that round? Honestly, not much reason to change this, except that a healing surge is fairly standard.
Lifetap: I'd suggest you give the benefit only to allies within 5 or 6 squares of the undead.
Now, for a few powers that I'd like to throw in:
Sickening Explosion
Sickening Explosion Necromancer Attack 13
A corpse within range quickly blackens, pustulent sores and lessions form over its surface, just as quickly scabbing over. The body arches in a horrible rictus, looking like a plague victim shortly before collapsing, a cloud of stench emanating from its body.
Daily * Shadow, Poison, Implement
Standard action, Area: burst 3 within 15 squares.
Target: 1 corpse
Attack: Intelligence vs. Fortitude
Hit: The target takes 3d6+ int poison damage and is infected with either Blinding Sickness, Mummy Rot, or Cackle Fever (your choice, and make an attack as appropriate for those diseases).
Miss: Target takes half damage, and is dazed until the end of your next turn.
Sustain: Minor action, and you lose a healing surge or 10 hit points.
Special: You may expend a healing surge once per round to change the disease produced by the cloud. You spend this healing surge when you decide to sustain the cloud.
Shadow Burst
Shadow Burst (Necromancy attack
The body you target convulses and blackens as shadow energy pours into it, then erupts outward in a cloud of darkness. Those within the darkness can feel their life leech away. You keep the cloud charged with your very life essence.
Daily * Shadow, Necrotic, Implement
Standard action, Area: burst 3 centered on a dead body within 20 squares.
Target: Anyone in burst
Attack: Constitution vs. Fortitude
Hit: The target takes 3d6+ int poison damage and treats light conditions in the area as if in Darkness.
Miss: Half damage, and target considers local light conditions to be Dim.
Sustain: Minor action, and you take 5 points of damage.
Special: If a creature is killed by this effect, you can choose to raise it as a minion until the end of the encounter. This costs you an additional 10 hit points or a healing surge. The minion does not count against your normal limit of undead minions, nor is it especially modified by any feats you have. (If you use the "minions gain a bonus based on necromancer's level" rules that I suggested, this minion gains none of those effects). Each time a creature dies to this spell, you can choose to expend either a healing surge or take damage equal to (10+ 5 per minion you already control through this spell) to raise the corpse as a minion.
I put in a "expend a healing surge OR take damage" option for most of the sustainable effects, just because I think that it works nicely for the necromancer's theme, and gives a bit of flexibility when low on healing surges. I don't know if the damage is too low, too high, or what... I haven't played it out yet. It might also be useful to change the sustainability option to something like "expend a healing surge or take 10 damage. If you are out of healing surges, you cannot sustain this effect, even if you are willing to take the damage." so that the necro can't just burn through healing surges, and still expect to keep powering his spells.
I'd also make it so that you can only use a body once, for any effect. You can't make it the target of a corpse explosion spell and then animate it, for example, nor could you use it to recharge your Dead Man's Cry, for example. However, since the resurrect ritual does not specify what condition the corpse is in (as far as I recall), you can still use that and speak with dead.
Also, I love the idea of that scythe! Perhaps give the Scythe the same benefits as a warlock's dagger (whatever it's called).
Also, for the skull implement... instead of having just a skull, maybe allow for a skull on a rod of some sort (perhaps even a femur), or maybe a shrunken head... it would depend upon the magic item, I assume, some would be just an unadorned skull, some might be a skull with arcane etchings, some might be a skull with gems in the eyesockets, and all the teeth replaced by precious stones... etc.
Speaking of femurs, perhaps you could use something like that as an implement? Give the necromancer the ability to use a staff made of the vertebrae of some humanoid (or the legbone of some giant), or a wand made of a titan's finger (or humanoid's leg).
A couple of magic skulls:
A few orbs could be easily modified into one, like the Orb of Sanguinary Repercussion, just have it be a skull and deal necrotic damage.
Most of the other orbs don't even require that much modification.
A lot of the Rod powers work well for the necromancer concept, as well, especially if you change them into a fetish of some sort (like a ritually decorated thighbone).
For example, change the rod of dark reward into something that gives you +1 bonus to armor whenever you raise a minion, or a minion of yours dies. Both bonuses stack, but won't stack with other bonuses of the same (Eg, you raise 3 minions, you only gain the +1... 4 minions die, you only gain +1).
The Rod of Death's Grasp works well, you could change it to take effect when you hex a foe, maybe reduce the ongoing damage to 5 (because hexing a foe has a much better combat effect than a warlock's curse) and temp hitpoints gained to the same.
Using skeleton-based staves (staves made of bone, or perhaps capped by a skull) would be similar, and many of the ones in the PHB can be easily converted. The Staff of Fiery Might, for example, could just as easily be shifted to Necrotic Might.
For the scythe-style necromancy image, instead of having a skeleton-based stave, have it be a skeleton-based staff. Instead of quarterstaff damage, it deals scythe damage!