Any hotels with an address in "Venezia-Mestre" is actually in the dull, modern, industrial suburb at the mainland end of the bridge over to the real, ancient Venice you came all this way to see.
Do not stay in Mestre! You'll spend more time and money commuting each day in an out of Venice proper than you will save.
No matter what kind of lodging you pick, if it's summer (a) try to get a room with air-conditioning and (b) even if you can't (or you can but have a hankering for some fresh air) resist the urge to open the windows to your room.
Venice is, I believe, the primary breeding ground for the mosquito population of Southern Europe, and precious few Italian hoteliers have discovered that newfangled invention called window screens. Keep the windows shut, or prepare to be bitten.
(Also, carry some bug spray for those romantic canalside dinners outside. Trust me.)
The Venice hotel tax: As of 2011, Venice began charging a Visitor Tax. This is the city's doing, and it is not a scam (just annoying). All charges are per person, per night, for all guests over the age of 10, and the tax is charged for stays of up to 10 days.
The breakdown is insanely complicated, but general you pay €1 per person per night per star rating.
So a couple (2 people) staying three nights (2 x 3 = 6) in a four-star hotel (6 x €4 = €24) would pay an extra €24.
Non-hotels cost a bit less: €3 flat for B&Bs; €1.50–€2.50 for most apartments and residences, etc.
Dec-Jan, you get a 30% discount across the board.
Kids aged 10-16 pay half.
Stays at hotels on the outer islands or mainland are a bit cheaper.
Some hotels have folded the fee into their quoted rates; others tack it on when you check out. Just be prepared.
Venice is way more popular than the number of beds it has, so while in the dead of winter you can often show up and find a good place to crash easily, the best rooms (and the best-value hotels) are booked well in advance for the summer months and the two weeks prior to Ash Wednesday (when Venice breaks out the fancy dress and masks for its famed Carnevale celebrations).
Same goes (though less so, and more at the chic and high end hotels) during the Venice Biennale art festival and the Venice Film Festival.
If you're looking for a hotel near a particular sight, just go to that sight's page and, in the sidebar on the right, you'll see a list of all the nearby hotels (with "Reid Recommends" choices preceded by a little RR icon: ).