What does an expansion in early december mean for the standard format?

There won't be a rotation with Mean Streets of Gadgetsan. The standard format rotates with the first set of each year. So it will be rotating when the set after the next will be released. Then all sets of 2015 will rotate out at once (Blackrock Mountain, The Grand Tournament and League of Explorers).

All is explained here: http://us.battle.net/hearthstone/en/blog/19995505/a-new-way-to-play-2-2-2016

How often will Standard format change?
Standard format will be updated with the first new Expansion released each year. When that happens Standard format will be updated to include card sets from the current and previous calendar year. The Basic and Classic card sets are always part of the Standard format.