WARNING: This could get long. I am very picky about house plans and have a lot of ideas. It's a dangerous subject for me. I could talk about this stuff for hours.
As far as styles are concerned, I tend to go for either Southern/Country/Georgian or classic New England stuff like Cape Cods or Salt Boxes. Victorians can be very nice as well. And if the house has a front porch it had better be usable, not some cheap 3 ft deep token that the builder can try to pass it off as an added luxury.
I never really cared much for homes with a master BR on the first floor. I know it's a big thing these days, but I prefer all BR's on the 2nd floor. I like the seperation. I also always look for plans where the downstairs bathroom/powder room is at least partially out of the main living area or not in the front hallway. I would never want to invite a guest into my home and have them walk directly past the bathroom. Bad form
These days, pantries are a must. Also, not necessarily a large home office but at least a desk in the kitchen area, or a small nook for a desk and computer. Like a command center (I've seen it called that somewhere before).
Laundry room on the second floor. Why bring the laundry downstairs to wash only to have to bring it back upstairs to put away? If it can't fit upstairs then a decent sized laundry/mudroom near the entrance to the garage is good.
Garage entries in the back, or at least not taking up the entire front facade. I want to see the house not a garage door, even if it is camoflaged or made to look exactly like part of the house.
On smaller 2 story homes a second, a back staircase is not very feasible but I always love houses with 2 staircases. There's just something about them.
Fireplaces. Wood burning fireplaces. I know the trend is to go with gas but for my money there is nothing like the sight, smell and sound of a wood burning fire in the winter.
Ample closet/storage space is a must. Families collect a lot of stuff over the years. Need a place to put it all.
A couple of examples of plans I will look at more than once...
DB-5521, 2092, DB-3156, GML-E-363A, 2100