I don't avoid early aluminium, I do have a preference for steel, and would love a Ti frame, but ally is fine if in good shape, and I love u-brakes.As the title says. It might be totally subjective, or pure taste, not trying to make a list of bad Vs good as many might be looking for exactly what I would avoid.
I start:
-U brakes. I loved GTs and their colours but didn't get one at the time because of that
-early aluminium
-suspension forks, regardless of vintage
-I prefer single colour frames, but won't completely avoid a multi coloured one if specially nice
As others have said, rust, dents or other obvious structural damage, cheap tubing, low end parts although some frames come with lower end stuff and carry upgrades well.
Stuck seat posts.