I think Cahill looked pretty good last year but has been mostly terrible this year, as you say, making lots of mistakes, a lot don't end up costing the team, doesn't make them less bad mistakes. He throws himself around a lot, gets out of position a lot and overall this season I think has had a lot of very poor games.
Terrible game to watch, Chelsea were abysmal and it seems no matter how much this formation doesn't really work and seems stale and lifeless, they keep using it. Balance, a team has to have a balance of different types of players, Mata, Lampard, Oscar and Hazard, will look great against a completely rubbish team but they have no natural movement or flow to their game.
Newcastle Southampton was a lot more entertaining with some good goals, quite a few fouls, a terrible haircut and a good result. Southampton, really hope they don't go down, by far and away the most entertaining team to get promoted, just the first 10 games of the season were epically awful. Since then its 17 games and 23 points, with 12 games to go I think they way they are playing it shouldn't be too much of an issue, WAY better than most of the teams in the bottom 4-5.