-Short: It's a relatively short game, taking 2-3 hours to finish (about 3 hours for your first game). This is great if you don't have endless time to kill and are petrified of more daunting wargames like Axis & Allies or Twilight Imperium which could suck up a whole day.
-Immersion: The flavor text and personality for each chaos god is excellent. Each chaos god has unique winning conditions and powers that fit his idiom. For example Nurgle plays a Rain of Pus card on a battle to gain defense. You have to alter your strategy to fit your particular character's strengths if you want to win.
-Pick up and play: The game mechanics are pretty easy to pick up and play compared to many other army combat board games; making it easier to bring this up with casual gamers who are intimidated by giant rulebooks and stacks of chits.
-Randomness: Good and bad. I like the randomness of Old World card events, but they tend to cripple certain players if they fall just right and sometimes they never recover. It does make every game unique, but that is not always a good thing especially when you win or lose at the whim of fate. I like how Slannesh's cards allow you to take over other players' units temporarily and Tzneetch can teleport units around-- they are both very fun mechanics that also fit the chaos gods' personalities.
-Figures: The plastic figures for units are great and a welcome change from cardboard chits (of which there are plenty in this game). The figures are also unique for each god's higher cost followers and are very detailed.
-Balance: Because the chaos gods are each unique there is always the question of game balance. There are two ways to win: get 50 Victory Points or advance your Threat Wheel (or the game ends when you ruin too much of the world and compare Victory Points, or Old World cards run out and the game wins). The problem is that each player's Threat Wheel advances with different conditions, some of which are easier than others, and some players have to advance it more to win. Nurgle really only can win by Victory Points and Khorne can only win by advancing his Threat Wheel. The other two gods are a toss up, but I have found that the Threat Wheel advancement is easier in general for both.
My first game I played Khorne in a 3 player game and it was ridiculously easy to win. Khorne's units are just much better than everyone else's and as you upgrade them it gets even easier. I don't like that only Khorne wins by attacking other players: everyone else is avoiding combat and trying to complete their own agendas while Khorne is picking fights.
The next game I played Tzneetch and we kept Khorne out because he seemed unbalanced. The problem was that the Old World cards that came up this game made Tzneetch's winning conditions much easier and through no fault of my own I was winning again. With four players it evens out a bit, but mostly because there are more people to gang up on the winning player when it unbalances.
-Lack of variety of cards: The power cards for each chaos god repeat a lot. The powers there are nice, but I would have appreciated a more varied deck. It stifles replayability.
-The Map: For a Warhammer game it has a pretty boring map. It is the Old World, but the regions are all generic with a number rating and whether they are populous or not. The map never changes and many random events always target the same map areas. Your units don't actually move: they just get summoned into a space adjacent to an area you already have a unit. When you run out of units you have to summon them out of other areas (paying the same cost as if you summoned them from scratch). So it really doesn't feel like you're moving around in a medieval world as much as plopping demons down on a cosmic chessboard.
-Too many cardboard chits with no place to put them all: BYOPB (bring your own plastic baggies).
So a lot of nit-picking criticisms but overall I like Chaos in the Old World. If you're looking for a light wargame with a lot of Warhammer personality that can be finished in a night then this is a fun choice. I wouldn't play this every day though as the strategies will get old. You will find that you like one chaos god's powers and winning conditions more than the others', and everyone might end up fighting to play one of them unless you assign roles randomly.