Here are some of the things I take for granted, but which I am likely to experience involuntarily during my lifetime:
0. Food Grit
I was reminded of this gratitude when eating some lettuce I had not washed thoroughly enough. In the bad old days you would find rocks in your beans (I remember my mom inspecting lentils for rocks), and when you accidentally bit down on one you could damage your teeth.
Milling processes have made even brown rice fairly soft on the teeth. Older generations ground down their teeth on grain husks.
Many people still put up with this, of course. I don't even want to imagine living in a desert region!
1. Dental Care
When my teeth hurt I can go to a dentist. It is not cheap, but dentistry has the tools to prevent me from losing those teeth. In the past your teeth would become loose and/or broken, they would fall out, and you would face great nutritional challenges.
This, of course, is still the case for many people who eat at our local soup kitchen, which is why soup kitchen food is uniformly soft. One difference is that even poor people can usually get tooth extractions when teeth become too infected or too exposed to tolerate.
2. Unspoiled Food
Related to the above: when I see that potatoes or lettuce have gone bad, I have the great luxury of throwing the rotten food onto the compost heap.
This is a mixed blessing, of course -- food wastage is enormous at every step of the food consumption pipeline, and retailers prefer to throw away misshapen food rather than sell it -- but I appreciate having a choice to avoid eating wheat that has been chewed (and defecated upon) by mice, or eating grains with weevils in them, or eating rotten produce because that's all that's left until the next harvest.
I was reminded of this lesson while reading Thomas Hardy's The Mayor of Casterbridge, where spoiled wheat factored into the plot. That novel was set in the late 1800s, not the dark ages.
3. Houseguests
Earlier this spring I got freaked out by two vermin scares: cockroaches at work and ants in my room. Thankfully neither threat materialized, but it made me realize just how lucky I am to live in relatively vermin-free surroundings. Occasionally I have to deal with a mouse. Given that I live in a basement there are hairy centipedes and spiders. Every summer the fruit flies assert themselves. But my normal expectation is that I can live in a room that is free of infestations, and in fact that I can sleep on the floor without a bed and not have to worry. These are great luxuries.