The midfield. I think we have an outstanding fully fit midfield, and our possession play (mainly thanks to Modric and Parker) is superb. Us having more possession equates to less pressure on the defence. Less pressure on the defence means they'll make less mistakes (even though mistake vs situation will remain the same). Finally being punished for mistakes is what most fans see. But take Ekotto over the last two seasons. I just sit there counting bad mistakes, but hardly any of them were punished (mainly down to poor crossing, or finishing, from the opposition). Sooner or later was going to run out and it certainly did against Arsenal.
We need to get back to basics again, and pick the right team. Parker/Modric in the centre is our key. Due to the excellent possession they both provide it's actually more defensively solid than playing two out and out DM's.