Let's say you cannot vent your dryer. You live in a condo in Boston or any other city. For LEED and PassiveHouse certification, its pretty common. Now you have really two options. Either you buy a washer with a separate condenser dryer (turns steam into water) or a washer and dryer combination in one unit. My strong recommendation:
Do this: Bosch washer and condensor dryer
Not that: Any combination washer and dryer
Why: First the dryer in a combo is 110v, so it is notoriously slow. Secondly the rate of repair is far better with a condenser.
If you have purchased a condenser set, run a water line from the dryer to the drain in the washer. The water will then be drained from the dryer therough the washer, so it does not have to be emptied (my drivers taught me that).