I think the best way to get an idea is to look at feature lists for fancy shells like zsh or fish. But in short there are a number of things a good shell can do to help to execute commands faster and more easily. Stuff like autocompletions which make you faster, and also make things more discoverable; fuzzy searching/matching; navigating command history; syntax highlighting which helps to spot errors, and helps to understand the syntax of the command you’re writing.