[Article: How to make a Good CTF Map]

First off, this is all my opinion. All of it. This is not a strict map making guideline; it's what I think makes a good map.
Let 'em walk

Any person should be able to walk from their flag to the other flag. They should not need to jump, grapple, etc. to get from flag one to flag two. Am I saying that there should be no grappling or jumping? No! McKinley Revival, for example, requires you use the grapple to get a good defensive posture, and to get the adrenaline, you have to jump, but you can walk to and from each flag. You should make the grapple a nessesary to get powerups and powerful weapons, but not make it so that you can't to the base.
Keep it simple, stupid!

Don't make your map a super complicated maze. I'm not saying don't have nooks and crannies for people to duck around in. Those what make a map fun. But don't try to put everything you know about mapping into one map. If you want a to show off you mapping skill, release a map pak. Don't stuff your entire resume into one map.
Put it over there!

Item placement can make or break a map. You should have some good weapons (Rocket Launcher, Chaingun) in base, but keep some really powerful weapons (BFG10K) in the crossroads. The BFG should never be in a base. NEVER! Also, don't put a quad two feet from and invul and a power shield. When you put a weapon somewhere, put a box of ammo right by it. One on each side looks good, by the way. And when putting a power shield somewhere, put some cells by it. But don't cluster all the goodies near each other. You can make hotspots where you can find a lot of ammo and a quad or something, because it'll make some interesting fights.
Stick to your theme

Ever see a map that has higly technological bases, but the perimeter is out of the middle ages? Dont do that!! When you pick a theme, stick to it. Don't make your simple simplicity bases be surrounded by overly-complex perimeters. The base-built-into-the-side-of-a-mountain theme is popular, and be pretty good if used correctly, but suck if used badly. The middle-age's-castle theme I like a lot, because you can make it more fun. You can make more hiding places and stuff.
Broken Mirrors

Symetery gaurentees fairness, but not always fun. A good non-symetrical map can be so much more fun then the same old mirror map. But make sure you make them fair. Making three hard to guard enterances to the red base and then one enterance to the blue base that can be defended by a guy with a rocket launcher is completely unfair. Make the same amount of entrances and exits to and from a base. Make sure there's at least two. Balance is crucial with every aspect of Quake2, and map making is no different.
Where'd you go?

Make lots of hiding spots and obstuctions. If McKinley Revival didn't have creates in the base, the BFG's power would increase. Make you map around what weapons are on it, and make it so people can dodge projectiles, hide from BFGs, and not get gunned down. But don't make it so that weapons are useless. You have to find the correct balance for each map.
Testing, testing, 1, 2, 3...

Test your map. Get a bunch of friends with different computers to test out your map. Make saure everything works, and fix anything wrong with it before you release it. The DTeam does this, and know what? They have some of the highest quality CTF maps out there. They test each other's maps on different resolutions, in different video modes.