It's wonderful to hear some positive reports on Venice in a wheelchair.

The city isn't, on the whole, very wheelchair-conscious - they just built a new bridge over the grand canal that has no wheelchair access! For a while, they proclaimed a system of lifts to assist in getting over staired bridges - on my most recent visit, they have abandoned and removed them (they almost never worked anyway in our experience).

I think Venice is one place where careful planning is absolutely essential.

As Katya noted, the water bus (vaporetto) is excellent - so if stairs are an issue, make sure your hotel can be accessed from the vaporetto stop without any stepped bridges (almost all bridges in Venice are stepped - thankfully many hotels are close to vaporetto stops).

Some of the attractions have put in some effort to become accessible - on our most recent visit, we noticed a lift to give access to the upper level galleries in St Mark's church (well worth the visit); the Campanile at St Mark's and at San Giorgio also have lifts, but there are some stair issues with getting to the lifts.

And a general piece of advice for Italy - if steps are an issue, and your hotel advertises that it has a lift, check that there aren't steps up (or down) to the lift itself (we've found this a few times).