Quality levels...
Tourney
Altus
Acera
Alivio
Deore
etc.
I read some comments here some prefer Acera at the minimum. The jump to Deore from what I read can be significant.

I noticed though if you look at bicycles at the same price like two $450 bicycles, if one has suspension forks and disc brakes, it could have Shimano Tourney on it (some people don't recommend) while another one with rigid forks and V-brakes could have Acera on it.

At the other extreme, where most people probably don't want to go, if you went with Falcon friction shifters for example, you wouldn't care too much about index shifting performance. Some distance cyclists on this site though say Deore LX will last longer than lower quality levels.