Design and materials are separate issues. Design involves roominess: whether the tent is divided, how much ventilation it has, how much rain coverage, and so on. You can be a pretty good judge of that yourselfyou say "three-season" camping, but my guess is you're really after a tent that goes from late spring to early fall. In terms of materials, tents that use aluminum poles and polyester flysheets are lighter, more durable, and more sun resistant than those using fiberglass and nylon. That is a generalization, of course, but not a bad one. The mysterious "D" to which you refer simply describes the shape of the door, which is usually mesh.
Eureka's Equinox 6 ($319; www.eurekacamping.com) is a fine all-around family tent. It sleeps six, albeit in a single room, and has a sturdy aluminum frame and nylon walls. Weather coverage is excellent, and the floor is an extra-heavy material for durability. The Eureka two-room Headquarters Tent (also $319) has a little less weather coverage but adds an interior divider so somebody (the golden?) gets some privacy.
Coleman makes some decent tents, too, that cost less than most Eureka models, largely due to their less-expensive fiberglass poles. The Galileo, for instance, is a two-room tent that can sleep up to six. Good weather protection, although this isn't a tent that could withstand a hurricane. Cost is just $174 (www.coleman.com). L.L. Bean makes some very nice tents, too. Bean's Big Sky Cabin Tent is really, really bigsleeping eightand has an appealing screened "front porch." It includes a polyester fly and aluminum and fiberglass poles. It's not cheap at $399, but it is an excellent tent (www.llbean.com).