I'm not sure they are reffering to the difficulty of the actual gameplay. It's more like sometimes it could be a little confusing for the casual gamer.
Gears of war and Half Life are very simple .. linear shooters .. very accessible even if they are not so easy at the higher difficulties. GTA is also very accesible. Lot of things to do but it is always showed very clearly where you should go on the map.
Oblivion .. easy enough but you have to think more and experiment a little to find your way through the game. Same could be said about eg. Bioshock. Casual gamers hate this. They want the game to hold their hands all way through.

What I am trying to say is that they can make the game both more accessible and more difficult than the first one. I don' tneed it more accessible but I don't see it as a problem. I am looking forward to hopefully a great game