I don't understand why somebody downvoted you, I've seen a number of languages that look like Lisp, but are based on vectors or arrays, and that changes the language drastically. So I'm wondering, too. I was trying to find out whether Janet programs are lists or something else like arrays, but didn't find it in the docs.
This does change the language drastically - source code is represented as tuples usually, although all of the core data structures have literal forms. This means writing macros is still easy, although certain idioms like consing are usually replaced by splicing, which is like unquote-splicing in Common Lisp but more general.
A list refers to a singly-linked list, while tuples are implemented as immutable arrays. The former is flexible in that multiple lists can hare structure, and prepending to a list is an O(1) operation that does not change the original list (consing). This property allows all sorts of interesting data structures which at their core are simply lists of atoms and other lists.
Janet on the other hand just uses tuples, which are easier to pack densely into memory so usually have better cache performance on reads (clever lisp implementations can sometimes can make lists fit densely in memory, but usually they take about twice as much memory as a tuple).
splicing is the like the spread operator in JavaScript or the splat operator in Ruby. This turns out to be very useful in a language without cons for manipulating tuples in macros, even though it is not as efficient as a cons.
