An ongoing discussion about making the Emperor's Finest become the table-top's finest!
1) Grey Knights have excellent midfield dominance due to two primary factors. They have deadly firepower within 24 inches because of S4/S5 Storm Bolters and Psycannons, available on all of their possible troop choices except Henchmen. Every unit has access to anti-infantry shooting (Storm Bolters) and excellent anti-tank shooting (Psycannons), though limited to 24". Psybolt Heavy Bolter Razorbacks extends this somewhat to 36", though not in enough volume to break out of their sweet spot. Psybolt Autocannon Dreads are one of the only ways that Grey Knights can challenge the far board edges from the early game.
2) Grey Knights excel as shooting, counter charge, and generalist units. Storm Bolters as standard equipment means that you can torrent with every unit, and at the sweet sweet range of 24". This allows you to move, shoot, and remain out of charge range for most things as long as possible. Their Nemesis Force Weapons also allow them to beat up on whittled down units and even put the hurt on fresh semi-elite units (Berserkers, Blood Angels, Grey Hunters, etc).
3) The new trend for 5th edition codex design seems to be to have HQs which unlock certain units as troop choices. GK has this and more. Draigo = Paladins, Crowe = Purifiers, Coteaz = Henchmen. This is good because Paladins, Purifiers and Henchmen are all workable troop choices. Grand Strategy takes this one step further by making almost any unit a potential scoring unit (even Dreads!). This allows great flexibility in list building.
4) Psybolt Rifleman Dreads. A Dreadnought with two Twin Linked Autocannon arms, with the Psybolt upgrade. This single Dreadnought configuration shores up the greatest weakness of 3rd-4th edition GK- long range anti-tank shooting. The Psybolt Rifleman Dread (AKA Psyfleman) has other benefits besides 4 twin-linked S8 shots. His Reinforced Aegis means that he can remain backfield while giving a passive defensive boost from Psykers, and especially psychic shooting attacks. Fortitude on a LD10 means that it's going to be very difficult to suppress these dreads with light anti-tank shots, unless you've got a Psychic Hood or a Rune Priest or some other anti-psyker powers in effect.
1) Current Grey Knights, like the previous edition, are an elite army (barring Coteaz Henchmen builds). Although you can run a list with lots of cheap Strike Squads, which isn't necessarily bad, the average 2000 point list will likely have 40-60 models. Grey Knights are capable of running one of the most elite lists out there as well: Draigo-Wing. You can have a nice 2000 point list with 16 models, and still make the opponent sweat with your tough-as-nails units. However, lower numbers of models means that your opponent will often take greater advantage of torrenting fire, your own mistakes, and board control. Part and parcel to this weakness is the fact that power armored Grey Knights are still T4/3+. This is no more survivable than a tactical marine. Poison, lasguns, multi-lasers, plasma, melta, etc, will still kill your expensive 20-30 pt models.
2) Grey Knights are generally not a combat army. Yes, they have Force Weapons, yes they have access to special stat increases (Hammer Hand, Nemesis Force Weapon varieties, etc), and yes you have options to field some true hard nuts (Paladins, Death Cult Assassins, Purifiers somewhat). However, they are also paying for their storm bolter, they're usually rocking 1A base, they generally do not have 2 CCWs, they rarely have any decent Invul save, and they lack easy access to fast and reliable delivery (Fast rhinos, cheap skimmers, open topped transports, etc). Although some debate this, I believe experience will show you that Grey Knights excel as a generalist army. Imagine super Tactical Marines and you'll get the picture. They have quite good shooting, decent combat ability against heavy infantry, and equal durability to any space marine. They will still die to Hammernators, Incubi, Nob Bikers, Nids en masse, and Furious Charging Blood Angel semi-hordes. Remember that you have Storm Bolters and Psycannons with unlimited ammo. Shoot it first, and then get into fights that are highly favorable for you. Let your enemy come to you unless they're long ranged shooters like Guard.
3) Weakness versus Anti-Psyker gear and rules. With such army-wide benefits from psychic powers, running into Runes or Njal can really hurt. The worst possible match-up for Grey Knights, comically, are usually other Grey Knights. Since Grey Knights are some of the best units to go and kill Psykers (sorry Sisters!), their special rules royally screw their own chapter when fighting amongst themselves. As in, when Grey Knights face Grey Knights, the unit that gets the charge will almost always win in a straight up fight. Also, anything that makes Perils more likely can see you killing off your sergeant, which means your uber-elite Grey Knights are now LD8.
4) Since Grey Knights tend to excel as a Hybrid Army (Vehicles and Infantry) they are also susceptible to feeling the full brunt of their opponent's firepower. With a fully mechanized list the opponent will have to holster their splinter cannons, bolters, CC troops, etc. When it's all infantry, their missile launchers and melta guns are a bit over kill for their cost, when infantry will almost always be rocking a 4+ cover minimum. However since Grey Knight's infantry shooting ability will encourage players to drop their troops off by turn 1-2, this means the enemy will usually be able to make use of all of their guns and units. For this reason, Grey Knights should usually be played quite aggressively, though not blindly so. You can't turtle up and pop Psycannon shots out of Rhino firing points all game and pray that Psyfleman Dreads will force your opponent out. Getting your infantry out of their vehicles needs to happen earlier for Grey Knights than most other mechanized armies.
Have fun reading; I'm sure others have far more useful advice to give!