What was exactly so bad about Sarkozy? I'm sorry for being completely uninformed before asking this.
That's a good question actually.
I think most of the hate comes from his personality : he's a very unlikable person, arrogant and slightly megalomaniac, fond of himself and likes to show-off (he appeared on the front page of tabloids, wearing luxury watch and traveling on yachts, celebrated his first election in a luxury restaurant, had a presidential plane built for him, and of course he married an ex top-model). He can be agressive too, telling people to "fuck off" in public.
He also used to have an apparent strict position on police actions and immigration, although it didn't really show during the last 5 years, until the last weeks where he courted the far right votes.
As for more political aspects, his most unpopular actions were a fiscal law that prevented rich people to pay more than a given ratio, and the retirement reform that increased progressively the age of retirement (although everybody knows such a reform was necessary, his opponents say it was badly handled).
On the international front, he got critics for receiving Qaddafi in Paris a few years ago, although he also had an active part in the military action against him later. There are suspicions of Qadaffi giving him money for his 2007 campaign, but nothing official yet.
In the end I think he's mainly paying for the current economic situation : rise of the debt and unemployment, downgrading from AAA... People are frustrated and blame him for that.
I don't think he was a good president, but he wasn't a bad one either. No worse than Chirac for example, but everybody loved Chirac as a person.