The Details
The Conservatives, contrary to popular opinion, were actually very good at managing the economy. Kenneth Clarke, as Chancellor, made some very prudent decisions regarding tax and interest rates, and so, when Gordon Brown came to power, he inherited, in a situation new to an incoming Labour Chancellor, an economy which was growing; an economy which had low unemployment; and an economy which had low inflation. To slate the Tories over their economic record is to go on a hiding to nothing, since these fundamentals cannot be argued with.
As regards sleaze, of course, there was a certain amount of it in the Conservative administration. Of course. Many ministers had become either complacent, and thought themselves above the law, or were so disillusioned by the poor opinion polls that they simply gave in, believing that it wouldn't matter if they were found out, since they would not get re-elected anyway. There is no justification for this - but it is something that William Hague has attempted to sort out. At least he is trying. Also, whilst recognising that two wrongs don't make a right, there is a certain amount of this in the 'shining light' Labour party too. I only need mention the names of Mohammed Sarwar (election fraud), Robin Cook (affair with secretary, arms-to-Africa) or Peter Mandelson (unauthorised loan) to back this up.
Another criticism often levelled at the Tories is that they are out of touch with ordinary people. This, also, was probably true. But, again, all I can say is that at least they are, now, trying. William Hague mentions this regularly in his speeches, all over the country, encouraging Tories to take more of an interest in the issues of the people, instead of lounging around at Westminster drinking brandy and smoking cigars. And this is having some success: the Tories made some quite incisive gains in by-elections.
What else?
Criticism has been made of the Conservative ideology. This, though, is more dynamic than some people think.