^^^
Bad map features:
Doors that anyone can open, but only open from one side
Usually ends up more of a detriment to the defenders than the attackers.
Destructible doors of all types
Are they ever going to fix that first round bug? Consider just making maps with no breakable doors what-so-ever. Then that problem becomes irrelevant.
Any possibility of ladder bridge straight to the flag
In the end, attackers spawn in 3 seconds and run straight to the flag across their ladder bridge, which takes 20 seconds. Defenders, meanwhile take 30 seconds to spawn. Even if they spawn directly on the flag, it's basically impossible for them to win under these conditions.
Defender spawn points directly on the attacker route
29...28...27...26...25...24...23...22...21...20...19...18...17...16...15...14 ...13...12...11...10...9...8...7...6...5...4...3...2...1... respawn! Where am I? WHACK DEAD
29...28...27.... FUUUUUUUUU
Low buildings near the flag
These are invariably used by attackers with ladder bridges to create drop-down points which the defenders have almost no chance of stopping. Defenders have to actually get up to the ladder (if they can) and then have a long enough weapon to destroy flat ladders (a lot of 1H weapons in particular can't hit flat ladders).
Good map features:
Gates with gate controls
These often give a team a second point to fight at and can make a big difference if the defenders actually re-shut the gate at a critical moment.
Tall walls with nothing on the inside to drop down onto
This forces attackers to create a down-ladder (or else take the long way), which gives the defenders something reasonable to try to destroy.
Enclosed keeps
This is a sure-fire way to stop a ladder bridge from going straight to the flag. At least they'll have to get off the wall and get into the keep rather than laddering directly onto an exposed flag.
Defender spawn points behind the flag or unreachable
Something I think should be considered are "barracks" -- rooms where defenders spawn which are unreachable. Once you jump down, you can't get back up. This would give you a lot more freedom to place spawns where you like without the problem mentioned above.
I'm assuming map creators get to set spawn points...
Really I wish there was a better map creation method. I looked into it but declared it was too complicated to even setup. So kudos to the people who actually went through that mess and figured out how to make maps, but a lot of the existing maps have seriously fatal design flaws and just aren't that fun. I can hardly stand to play siege anymore just because of the maps.