Thanks for the comments, Karma, and to everyone who played a map that clearly needs some work. I was surprised (and gratified) at the full time you all dedicated to trying it out. You guys are great.
The game test really just flooded me with input. Long post follows.
Respawn rooms:
I noticed everyone initially ran out of the 'back door' of the spawn rooms. That surprised me, but exposed assumptions I built in my head as I built the map. ('I' new where I should go, so I assumed everyone else would too.

)
I'm hoping that if I turn the individual spawn points 90 degrees then people will see the capture point sign and head out to the ball field first?
I need to increase the size of the spawnrooms, add another door (or two?), to help avoid them getting shut down by campers.
Unused space/outfield:
I think I'll shrink the accessible areas outside the stadium. Game-wise, no one really needs to go WAY out in the parking lot, or over the outfield wall. That will avoid people wasting their time out there.
Play style/team balance:
I'm realizing that the core concept (capturing points as in baseball) is awkward to forcefit into TF2 gameplay. It turns the infield into more of a deathmatch area, rather than team play. And it robs any value out of controlling the outfield areas.
An obvious solution would be to move the CPs outwards. But that goes against the (admittedly flawed) core concept of the map.
Perhaps if I:
- place some health/ammo out there, but close in to the infield
- add some cover from which to fire into the infield
- build some patterns into the stuff in the outfield (creating attack/defend routes)
- and increase the time required to capture the bases
that all that will give some value to controlling the outfield, and maybe help Red defend?
Red clearly needs some help here. Setup time is a clearly missing, a glaring omission. I think if I also increase the time needed to capture each base that blue will meet more resistance. I had them set at 5 seconds for testing. Oops. Maybe, in base order, 15, 20, 25, 15?
Doors:
The spawn doors actually open pretty quick (400?) I think I have their triggers too small, so some players were bump into them anyway. (Another reason for me to increase the size of the spawn rooms.) The doors also raise back up too quickly, trapping the next player. I need to add some delay there I think.
Directions:
Signage is needed, both for game play and atmosphere. I've been learning as I go, and figuring out how to create textures/decals/overlays is on my list. Until then I'll need to use stock arrows, and more of them.
Bad texturing:
I've only recently realized that Hammer contains textures from HL, etc, that don't work well in TF2.
I'm also cleaning up my floors. ("Why are there parking lot lines on the red spawnroom floor?") I've already got them sorted out for the next version.
My favorite overheard comments during game play went something like this.
Player #1: "Why are the CP's so close together?"
Player #2: "Because this is baseball!"
Player #1: "But why are there train cars all over the outfield?"
Player #2: "Because this is BASEBALL!"
I laughed so hard at myself over that, because those two guys were SO right. Thanks, whoever you are!
Why trains? When I first built this, the wide open outfield was obviously a sniperfest. I was trying to think of some way to break up those sight lines. My wife suggested something from a movie she saw (Brewsters Millions?) where a minor league stadium had train tracks (and sometimes trains) running through the outfield. I meant to build tracks across there, but didn't like the available stock track selection. So I built that skybox crane as an excuse for HOW the train cars got out there. My only excuse for WHY is an attempt to break up sniper sight lines into the infield.