Venice is one of the most interesting and lovely places in the world. This sanctuary on a lagoon is virtually the same as it was six hundred years ago, which adds to the fascinating character. Venice has decayed since its heyday and is heavily touristy (there are slightly more tourists than residents), but the romantic charm remains.
The summer may be the worst time to visit: it's sometimes very hot and often humid, the canals usually smell, there are mosquitoes and occasional infestations of flies, and there are more tourists than usual. Spring and fall are probably best, a compromise between temperature (expect 5-15°C in March) and the tourist load. Between November and January, you may manage to feel you have Venice all to yourself, an interesting and quiet experience. Beware of the weather during the winter months: it can be quite cold, windy, and damp. Fog is an additional hazard if you are driving in or out, doubly so in the unlikely chance that you will pilot a boat. That said, if you've never been to Venice, it's better to go in summer than not to go. You won't regret it. Many cities are far worse in summer, and Venice has no cars, hence no smog.
Because Venice is on a lagoon, the water plays a crucial role in transportation. Whichever way you arrive, the last part of your journey will be on foot from the nearest waterbus/watertaxi jetty. If you need to carry or wheel bags along the narrow streets, bear this in mind when choosing your hotel location and route to it.
If you want to get around a bit more quickly, there are numerous vaporetti (water buses) and water taxis. The vaporetti are generally the best way to get around, even if the service route map changes frequently. If you are going to be in Venice for a few days visiting, it is a lot cheaper to use vaporetti than private water taxis. If you want to have a romantic ride along the canals, take a gondola ride, although they tend to exist for more scenic purposes, rather than getting people from point A to point B.
Venice has some wonderful restaurants, featuring the cuisine of the Veneto. However it is widely regarded that the restaurants in Venice serve food of a quality and in quantities much lower than anywhere else in Italy. The pizza in Venice is well known as being the worst in Italy (It is a more southern italian speciality). For Americans, you can find a place called Quanto Basta pizza that serves an American-style pizza with pepperoni and french fries. Rule of thumb: if there's a waiter outside pimping for business, it's probably best avoided.