no matter what time of year you go, it'll still be amazing! I went in may this year, the weather wasn't perfect, but it didn't matter, i've never been anywhere so beautiful in my life.
depends what youre asking about? the weather will be cold and it can feel damp so wrap up warm. it'll be less crowded than the summer months, so thats a plus (havent spent christmas there so i cant say about that).
as loukr says, its probably the most beautiful city in the world and is great anytime of the year.
I went to Venice in February. A couple of days were cold and misty but other days were gloriously sunny. But as someone else said, it doesn't matter. The city is beatiful at any time - even in the mist which makes it very atmospheric. The sunshine brings out the vibrant colours.
The advantage of going in winter is that you don't have the hoards of tourists to fight through. Everything is relaxed and the locals have more time for you such as in shops and restaurants.
Watch the prices in tourist spots though! A coffee in St. Marks Square is the equivalent of 14 pounds! Buy things like that in the smaller places tucked away in side streets. Forget the gondolas though. We were quoted about 75 pounds for half an hour!