Lack of funding. Specifically, a breakthrough required in public opinion which currently expects to receive £30,000 a year kidney dialysis or £21,000 herceptin if needed, but not to offset the costs through taxation. The failure to accept that the demands of the NHS have changed so much in 50 years as to be unrecognisable.
And I second the mental health cry. Most mental health drugs have hideous side effects and are often just shoved into mental health use after someone tried to discover an anti-obesity drug and found that it made people depressed/sedate/hyperactive. And there is a shocking lack of research in this area. (That's a rant from my mum, who's a mental health lawyer.)