Thanks for all the answers guys, I really appreciate it!
A couple of the frequently-recommended brands, Ryobi and Ridgid, don't seem to have a big-torque 1/2" impact wrench in the line-up. I'm one of those guys that is gonna need to knock a Honda crank bolt or an axle nut loose here and there, so a ~300 ft/lbs. impact does not do the trick. I also noticed the "catch" (sort of) to the very nice Ridgid lifetime warranty--it only applies to the batteries that come in kits with tools. Maybe it'd be worth it in the long run, but I'd end up having to buy sets with tools I don't need to have sufficient lifetime-warrantied batteries.
A couple of you mentioned that the lawn stuff (string trimmer, hedge trimmer) would require a different set of higher voltage batteries. Dewalt, at least, has a pretty full line of yard stuff that runs on their 20v Li batteries. Makita has gone a different direction and sells several yard tools that require two of their 18v Li batteries at once. You turn two 18v batteries into one 36v. Milwaukee basically hasn't got a yard tool line now, but they're releasing one next spring. It's a bit of a gamble, but frankly I have a lot of faith that Milwaukee will get yard tools right on the first try.
Pricing on Milwaukee, Dewalt, and Makita tools are pretty close to one another, but Makita does not appear to offer a kit with a reciprocating saw, one battery, and a charger--their only kit comes with two batteries, so ~$100 more expensive. Now, that's fair, but for my first piece, likely not bought on sale, I'd rather spend $250-300 rather than $400, even if I do have to stop work once the battery dies.
So, I think I've narrowed it down to Milwaukee's M18 system or Dewalt's 20v max platform. The Milwaukee stuff feels a little nicer, a little more solid, and seems to have a good reputation. The Dewalt line is a little cheaper, seems to have more tools that I can see myself using. Dewalt also has the most representation at most consumer stores, seems like the easiest to find used as well.
I do think that Round 2 is going to be the M12 stuff from Milwaukee. The form factor of their 1/4" impact, right-angle impact, and a couple other things is so compact that I think you could do things--get to nuts and bolts--that I've previously never considered using a power tool to do.