This is an attempt at building a casual play Dimir deck using only cards from Gatecrash. You won't win a lot of matches playing against a broader card pool.
Obviously, if you are interested in building a better deck, you can broaden you horizons by using cards from other sets. Dimir, however, seems to have so many building options, it does seem possible to make an effective deck with only cards from Gatecrash and it gives you great experience with some of the new cards. I purchased my cards on MTGO for very little money to build this deck.
Let's walk through the deck. I included Call of the Nightwing. Putting call of the Nightwing on an unblockable creature is an easy way to generate small flyers fairly quickly. Call of the Nightwing also works well with Sage's Row Denizen. Every time Call of the Nightwing creates a new token with Sage's Row Denizen in play, you get to mill your opponent.
Consuming Aberration can get to be a very formidable threat. Unfortunately, it costs a lot to cast. It also lacks many of the attributes which would make it a game ender, like trample or evasion, but it is still too large to be ignored and force the opponent to block. A much lower cost option, the Wight of Precint Six is also included. The Wight of Precinct Six can grow quickly once the milling starts- so start milling quickly.
Deathcult Rogue provides some qualified evasion which is necessary for maximizing your returns on cipher cards. (Invisible Stalker is much better, of course.)
Grisly Spectacle and Killing Glare provide for easy removal. The fact that Grisly Spectacle mills makes it work well with the other cards such such as the Wight of Precinct Six or Consuming Aberration. Killing Glare can potentially cost much less and like Death Wind before, it can provide turn 2 removal on a low cost creature. While Death Wind is necessary when dealing with indestructibles, Killing Glare can remove zero power creatures common in Simic on Turn 1. Killing Glare scales in power as the game goes on to deal with larger threats at greater cost.
Mortus Strider is a fairly useful card. It works well with Sage's Row Denizen and Undercity Informer. Mortus Strider is a great card that can be sacrificed and is good for chump blocking. Although the recasting of Mortus Strider is higher than the old Reassembling Skeleton, if you can get some extra milling out of it when you recast it, the casting cost does not seem too high.
Nightveil Specter looks like a bat to me. Nevertheless, it is a real threat that most opponents will want to deal with quickly. Unblocked, Nightveil Specter can exile the top cards of an opponents library and, if you have the mana for it, you can cast the exiled card. The second part is the tricky part as you'll need the appropriate mana. I've added some Thespian's Stages to try to fix mana issues. This, however, makes Nightveil Specter harder to cast as you need three-colored mana to cast it. Overall, Nightveil Specter is a great card that does force you to do a little bit of mana fixing and can be difficult to cast.
Paranoid Delusions is a great mill card that takes its toll on the opponent quickly. I really like the low casting cost and in decks with a lot of evasive creatures, it definitely works. I see others slapping this on an Invisible Stalker routinely.
Psychic Strike provides you with some countering ability. I generally don't like countering spells, but I do like the guild trend of providing some additional benefit. In this case, you get to mill, so you still feel you've accomplished something positive in addition to denying your opponent the use of a spell.
Sage's Row Denizen provides passive milling really well. It's like a Hedron Crab with creature-fall.
I've already discussed the benefits of using Undercity Informant with Mortus Striker.
The side deck provides for some different options. Aetherize is great for disrupting opponents attacks and works well against token decks. Duskmantle Guildmage has a lower casting cost than Consuming Aberration and can accelerate the mill slightly. I don't feel it is great value, but it might be useful if you need to get creatures on the board faster.
Glaring Spotlight makes an excellent sideboard choice. It can provide your creatures with hexproof if you need it or make a hexproof creature targetable.
Hands of Binding is one of my favorite new cards. It has an extremely low casting cost and can lock down the opponent's creature. Hands of Binding also works if you are dealing with difficult to remove creatures (Indestructibles) or those that power up too quickly for Killing Glare.
Finally, I've thrown in some Midnight Recovery for recursion. An Elixir of Immortality is normally my go to choice for recursion, but again I'm trying to build a deck using only Gatecrash cards.
For a deck that has serious design limitations, I think it works pretty well and it gives you a great feel for the new Dimir cards and mechanics. The cards are very cheap on MTGO and most of them will probably only get cheaper. Dimir is already looking very good and will probably only get better.