IMHO, a good long distance frame is one that you can ride a long distance on and still like it at the end. Generally speaking, while lighter is nice, it isn't as important as in road races or crits because it's about keeping a steady pace (whatever that pace may be) rather than needing maximum acceleration to get into the breakaway or sprint for a prime or whatever. Likewise, while aerodynamics certainly matter, it is less of a priority over comfort than in a road race (even in a road race, the balance between aerodynamics and comfort is different than it is in a time trial).
Referring back to earlier posts, keep in mind that while fit and geometry are not the same thing, they are definitely related. People think of seat tube angle, for example, as falling under "geometry" because it's listed on the spec sheet, but it really comes down to the relationship of your feet to your butt, which affects the angle of your torso, which might affect your desired handlebar reach, which is related to stem length and weight distribution, which is again related to top tube length and handling, which is related to front end geometry, etc, etc, etc. It's all related.
And it's also related to technique and body position. Many cyclists benefit from having a professional fit, whatever that means, or even a coach who guides them on better posture and technique at the same time as addressing the fit. Many don't need it, or don't benefit, or don't get the right person. Personally, I've always preferred to fit myself and make gradual tweaks over time and for different things on different bikes. I think there's a lot to be learned from trial and error and experimentation.
There's also often a lot to be learned from other cyclists. Some issues may be more easily diagnosed by an astute companion you ride next to for six hours than by a fitter in a shop who sees you hop on a trainer for a few minutes. Try and work thinks out logically for yourself, because not everything you hear (especially from other cyclists) even makes any sense and conventional wisdom isn't always correct. But if you have buddies who are especially good riders, pick their brains from time to time.
