Only skimmed the posts, sorry if I repeat...
The key word in the object of mapping is balance. There are so many things that need to be balanced because too much of one thing or another will crap up the map.
- Size: Some XMP maps are too big, where even with vehicles you have to haul ass between objectives. The only real example I have of over-small maps being a problem is in the Onslaught maps with only one node. For 3v3 they work but in 5v5 it gets crowded, 6 or more on a side is just spammy. TOO small.
- Mobility: Objectives must be spaced such that if a person is stuck because some lamer took a harby and drove off by himself, it's worth his time to go on foot. Something else that helps ground mobility is having spawn points in key areas so respawners can get into the action quickly. At the same time, it must be open enough that there is a reason and an advantage to use vehicles.
- Energy: There must be enough of it so that one team is not at an extreme disadvantage (perfect example: Lowlands. Your team NEEDS both in order to do anything effective), but not so much that ppl can deploy-spam without any worries (I personally have made a career of making the Nakoja caves into impenetrable fortresses)
- Recharge: Again, if you are all alone, or on a lame team, or on throne duty, it helps to have supply stations near (not necessarily at) the base, but don't litter the countryside with them. Again, make the players' supply packs have some value.
- Indoor/Outdoor: in order for all 3 classes to be effective, you need areas where the gunner can pwn with close-range/splash, and open areas for the ranger to roam. The tech of course rides the fence, so if you cater to these two, you will also take care of the tech by default.
- Access: only one way in/out of a base (Xaemia) can cause ridiculous bottlenecks, but having a node out in the open, or with TOO many ways in, will be impossible to defend. Bases and other key strategic points should be tough to take, but also tough to defend. Look at Garden: three entryways and two teleporters; look at SunsetBeach: two entryways and two teleporters. Both have multiple routes in and out of the base. Two GOOD defenders can hold these bases fairly well, but one good run on the base, or a lack of energy, can make their lives every bit as miserable.
Terrain: flat boxes are boring sniper-whore-fests, but huge rolling hills and narrow canyons would make driving a pain. Build in variety but keep in mind that a little goes a long way. Remember that terrain is part of the obstacle between objectives. Use it as part of what spaces them out and makes them strategic (example, the broken bridge in Alcazar, or the raised roadways in Sirocco).
Eye Candy: again, flat boxes are boring, but a huge zig-zag pattern of trees will piss off your jugg drivers. USE the trees and etc, MAKE them part of the strategy. Trees can obscure a sniper's view just as much as a giant wall. Use them to break up open areas. You can always go crazy in peripheral areas of the map if you want to add "spice" but be mindful when placing things where people will expect to be travelling.
BALANCING these things is what makes a map good. Do all these things and ppl will say "wow this map rocks" and maybe not even know why, not really be aware of the risk/reward factor you built into each of these areas. THAT'S IT! Think of it like a golf course: risk and reward. Make it tough, but fair, so that the n00bs can get around okay, but reward the vets for their skill. But get any one of these areas wrong and it may stick out like a sore thumb for ppl (lots of ppl grumble at lowlands because of the energy shortage, even tho many other areas are well-balanced).
Best thing to do is lay out your map before you even open the editor. Know the theme of your map, then start fleshing things out. Start with gens and spawns, and work out indoor/outdoor areas, open/obscured areas, where to place the manned turrets, etc. Knowing how ALL THE PIECES fit together will save you a lot of time in the long run, like when you get 2/3 done with your map then realize, oh crap, my map is completely flat.
My $.02.