Just some random comments, from some of what's been said here:
Yeah, I was thinking in terms of being more than 1200' AGL, getting above Class G. With the proliferation of towers of various kinds, I don't do much flying below 1200' these days. There are an amazing number of up to 2000' AGL towers throughout the US, and although they're all supposed to be lit, they're still hard to see if there's the slightest haze. And of course, there are still a bunch of unlit, unmarked towers that extend to 200', in spite of many states passing laws over the last few years mandating lighting and marking of anything over 50' tall.
Exception: One year my OSH trip included a buddy who was a C130 navigator with the Wyoming Air Guard--probably the sharpest pilotage navigator I've ever flown with, with many thousands of hours of low level navigation experience. When we left OSH, it was barely VFR, with thicker stuff above, and we couldn't get an IFR clearance until after Madison. He suggested that we keep it low, and with sectional in hand, he'd keep us from running into towers and dodging airport areas. So I leveled off initially at 1800' MSL,1000' AGL. I can't quite fly as accurately as he could navigate, but I remember one of his navigation calls: "There are a series of towers ahead. Turn left 4 degrees."

Mostly I prefer flying long distances at altitude, especially in the summer--cooler, smoother, and in the mid-levels around 8-10,000', not much traffic, as GA traffic is often lower and of course the jet/turbine traffic is higher. Eastbound, there's usually a stronger tailwind at altitude.
On how usable ADS-B is in the mountains, I've had ADS-B In for 3 1/2 years; I've only had ADS-B Out for about 11 months, and much of that time my airplane was down due to waiting for the new exhaust system to be built, and then I lost several months when the FAA pulled my medical until I got it back. So I can't talk much about TIS-B traffic in the mountains, but I can talk about FIS-B weather. Keeping in mind that the ADS-B towers are pretty prolific in the Colorado Rockies, while I can't get ADS-B weather on the ground, I can get it at relatively low AGL altitudes. Example: coming out of Marble a couple of years ago, I started picking up weather about halfway between Redstone and Carbondale while still in the canyon, not yet to 10,000' MSL. I think that's about 2500' AGL or thereabouts. From that point all the way home, I had good weather depiction. I get pretty good FIS-B weather at pattern altitude at both KGXY and KFNL, the two airports I most frequently use.
On some errant pilot not looking outside to see a brightly lit-up airplane, I think that's a fundamental problem with the combination of TAA and fancy autopilots. It's too easy to program a sophisticated autopilot to do its thing, climb up to and maintain some pre-selected altitude, while the pilot pays attention to something else, like his lunch or his passengers or some actual pilot-related chore, other than looking out the windshield for traffic. To some extent, we're all guilty of not looking as hard as we should at times. Granted that mid-airs are rare, they do happen, especially near airports and nav sites, wherever airplanes tend toward funneling into the same space. We like to think that our airplanes fly slow enough that it's not our problem, but there is an abundance of much faster traffic out there, so that closing speeds are still amazingly fast at times. Have you ever thought about how little time there is to react when closing speeds are around 400 knots (light GA at 125 knots, biz jet at 275 knots)? An airplane that is 5 nm away (barely visible) will be upon you in only 45 seconds! Even two light GA airplanes closing at hypothetical speeds of 125 knots each only have 72 seconds to avert a collision.
We can mitigate the danger somewhat by maintaining our own vigilance, by using flight following, by turning on our lights, by flying at correct hemispheric rule altitudes, by flying a little off airway centerlines, and (horror of horrors) making sure our transponders and ADS-B Out equipment are working correctly, but nothing we can do will totally eliminate the possibility.
I admit to having some comfort with the traffic reporting I now receive, although I don't yet have much experience with it. My new KT74 transponder is ES, so I get 1090 traffic there which appears on the 430W screen, and my Stratus 2 pulls in the ADS-B In traffic on both 1090 and 978 and displays it on my iPad Mini. My iPad Mini also shows conflict warning windows, and I get verbal conflict warnings through my headset from both sources. That's about as close as I can get to bubble-wrapping my airplane, I think.
On radar not seeing primary targets, many, MANY years ago I toured the ARTCC at Longmont. I don't pretend to know much about radar, but I noticed that if the controllers had the screens dimmed so that transponder equipped airplane targets weren't excessively bright, which was pretty necessary to reduce glare in that dark windowless room, primary targets (airplanes without transponders) were barely visible at best and disappeared at times. This was back in the early days when people were still carping about installing transponders, so there were a whole lot more transponder-less aircraft back then. I suspect the equipment is better now, so that they might not have to adjust the intensity quite so much, but it's certainly not something to count on.
Cary