My basement is unfinished and I have a large concrete wall. Can (should) I drill some small holes in the concrete to hold up a mirror or two?
Are there any serious downsides to drilling into the wall an inch or so?
Shouldn't be a problem at all - look in your local hardware store for masonry bits and concrete wall anchors.
Done exactly this last week. Have a basement with concrete walls and tiles installed on them.
I used tapcon screws. They came with a proper size concrete drill-bit for the screws.
Started off by marking where screws would go with a pencil. Then drilled very small pilot hole using smallest drill-bit I had. Having it made the tapcon drill-bit stay in place instead of sliding on tiles. Screws went in the hole tightly and hold mounts securely
It depends, if the basement is a prebuilt tub like construction it might void your warranty. I would check with your construction company.