Aside from recruitment, which was have proven to be woeful at, and management which we have tried and failed so far this season with three separate men, then you need to look at the actual make-up of a team.
Goalkeeper - We are sorted in this department
Full backs - Our options are majorly inadequate. I'd go as far as to say easily the worst in the league at this point in time. Baines is finished, Kenny is very raw, and the only other full back in the squad is Coleman, who was apparently training start of December but has since mysteriously vanished.
Centre backs - We spent big on Keane and he's proven to be an absolute horror show. He's played alongside Holgate, Williams and Jagielka and looked awful in virtually every game. Williams has shown a bit of a revival recently, but before next season starts we arguably need to sign two starting centre backs. Mori will have been out for a year and a half and I don't expect him to be here next season in any case. Holgate is average at best, but is young and may improve. Keane, Holgate, Williams plus two new starting centre backs may be ok to get us through next season.
Defensive midfielders - Supposedly Schneiderlin and Gueye. If we look at them individually then we know for a fact that Schneiderlin is the most hated player amongst the fans and with good reason. Nowhere near good enough to be starting and has a terrible attitude. Gueye can tackle, but that's about it. He cannot pass, cannot head, cannot attack (he scored other day, but for every goal he messed up about 20 times as many attacks with his rubbish shooting and decision-making in the final third). We need Schneiderlin booting out and a new DM brought in.
Creative midfielders - Is this where we put Klaassen, Davies and Rooney? We know Klaassen will be sold asap, and Davies has gone massively backwards this season. Rooney is just so-so, and next season his legs will likely have seized up completely. A major concern then, seeing as we literally have nobody we can count on here.
Attacking midfielders - Sigurdsson, who has been poor this season. He's our only number 10 option since it became clear that Klaassen is crap and Rooney doesn't have the legs for it. I think Sigurdsson would come good if he was surrounded by quality, but, well, he is surrounded by trash and perhaps it's no surprise he isn't able to do much.
Wingers - Mirallas, poo. Bolasie, poo. Lennon, works hard but is poo. That's all we have. Shambles.
Strikers - We have Niasse who everybody would love to come good but really knows that he's terrible. DCL may be good in the future, but as of now all he can do is win headers and sometimes hold the ball up. For a striker his shooting and goalscoring is pretty awful. Sandro is just no, and that's the total of our options.
So all that together, we are lacking majorly at full back, centre back, defensive midfield, creative midfield, attacking midfield, wingers and up front. I guess that's where the problems lie. But where do they stem from? Go back to the start of this post... recruiting. Our squad has a value of over 200 million (estimate, but it can't be far off, or maybe even more). I think even a poor manager would get us far higher up the table if that money would have been spent wiser. Instead we've been forced to play half a season with a relegation-material squad.