Exterminatus_Extremis

Traumatize. That is all.

I would say take out 2 of the Grisly Spectacle, if not all 4. Depends on your metagame situation -- in my circle, creature removal doesn't help a whole lot. You probably won't get much for the amount of mana you sink into it. A few more lands would be a good idea considering the size of your deck. For efficiency's sake I calculated that a deck (assuming a bell-shaped mana curve) should have around 42% lands or mana-producing permanents. In short, for a 60-card deck you would have 25 lands, for a 70 you would have 30.
You only have 22 lands for a 67-card deck. If you remove 2 of the Grisly Spectacle and add 7 lands, you would have a perfect 70-card deck ratio.
Again, this might work a little differently for every deck, but since I started this system I have never had mana problems in my decks.

Overwhelming consistency is key. Try to figure out what the most important cards in the deck are and build the deck around those. You should have at least three of each the cards you would use the most (not including legendaries; never more than two of those) and beyond that, maybe two of the support cards. For instance, your primary milling capacity comes from sorceries and instants like Mind Sculpt. Include more of those like Cerberus said, and leave out the loss of life stuff. Traumatize is your friend, unless you're going for standard. I can't tell since this deck is based on the Dimir starter deck and hasn't been adjusted very much from that template.

It seems kind of inconsistent. You would need every single mill spell you have to go off in order for this to work quickly, and the odds of even having all of those is fairly low. I'll grant that you can play pretty much everything for two lands, but I don't know if there's actually enough mill to make this feasible. Your instants combined get you exactly 60 mill and that's if you get all of them out and pay the flashback on Dream Twist. The Curses of the Bloody Tome and Shriekgeist add a bit more reusability but are likely to be destroyed as the game goes on and the opponent will probably have flying. This deck would work if you could keep your opponent from getting any kind of field advantage but once they do, it won't last long.

TL,DR; Add more mill and maybe more creatures to keep you alive and milling longer. 4 fog banks are probably the best bet here.

I'd put Dissipate in, but I use Cancel because that's what I had lying about. I'm heavy on the cards because for this particular deck, having more of what you need means having what you need when you need it. Excess of the combo pieces allows me to rebuild the combo if there's a boardwipe.

I really can't say I've found this to be slow. I consistently get 5th or 6th turn win conditions (not that I immediately win; I play casual with four or so people) and can usually pull an actual win out by 10th turn. I didn't put in any point damage creatures because that isn't the focus of this one. Primordial Hydra and Slime Molding exist solely as intimidation and an alternative win condition, not as the focus of the mana combo.

Needs more Aura Gnarlid.

For instant fun add a couple of Venom. It's an enchantment that gives Deathtouch WITHOUT needing to deal any damage. It's perfect for a deck made of walls, because walls tend to have 0 power.

Replace Leyline of the Void with Keening Stone. My mill deck has that as the ur-Win Condition. Once you've Traumatized someone once, you can Keening Stone them and it's over.

Green/White
Uses Aura Gnarlid and the better (cheap) enchantments to create a playerkilling machine. Once Aura Gnarlid is out with Canopy Cover and Ancestral Mask, there usually isn't any quick way to stop the deck, and even if it's exiled or whatnot, there are always more Aura Gnarlid.
Plus the Melira/Phyrexian Unlife combination.

http://www.mtgvault.com/ViewDeck.aspx?DeckID=302095

For extra Hedron Crab (for fairly cheap, too!) try the Prototype Portal/Phyrexian Metamorph combination. That can give you limitless Hedron Crab.
Then, landfall bomb using Oracle of Mul Daya and Terramorphic. The potentiality is 4 lands out in one turn with, say, 5 Hedron Crabs on the field. 5x3x4=60 mill instantaneous, enough to take out most decks. If not, then more Crab copy.
For a more long-term approach, use Khalni Heart Expedition to build up to a landfall bomb. That gives the potential for (with a terramorphic expanse thrown in) 10 lands out in one turn, meaning you don't even need all four Crabs out to completely mill a 60-card deck.

As a point of interest, a few walls with reach or flying would help you survive long enough to put any of this into place.

The classics: Death Baron, Lord of the Undead, and Undead Warchief.

Try adding something like Forgotten Ancient or Scute Mob into the deck. Those are very powerful, very cheap +1/+1 counter monstrosities.

You should stick one or two Asceticism in there. That would be a very good enchantment for this deck. You could probably take out one or two of the mana producing creatures like the Elves and Birds of Paradise and replace them with a couple of cheap unblockables, since Asceticism gives all of your creatures hexproof. There's also Mask of Avacyn, which is a very nice, cheap and reusable hexproof.
If unblockable isn't your style, you might also invest in some vigilance to keep your frequent attackers (like Geist) untapped to block with.