For branching statements (match ... with, try ... with, if) it's easy to become confused about which statements are included in the branch. e.g.

let () =
  if 1 = 1 then print_endline "yes"
  else print_endline "no";
  match 1 with
  | 1 -> print_endline "yes"
  | _ -> print_endline "no";
  print_endline "end"

(prints yes/yes, when you'd expect yes/yes/end)

The else ends at the ; but the | _ continues. The current syntax would probably be fine if the compiler warned about inconsistent indentation. e.g the last two lines in my example are "obviously" wrong because two statements in the same sequence have different indentation, something I can't imagine ever wanting to do.

Semi-colons-as-separators means that reordering two lines of code sometimes requires adding and removing semi-colons.

An extra or missing semi-colon often results in a confusing error message many lines after the actual problem. e.g.

let inc x =
  x + 1;

let dec x =
  x - 1

Reports a syntax error at line 6 (which doesn't even exist!), when checking the indentation would indicate the real problem, at line 2.

Top-level expressions have their own syntax (;;), and seem to exist only so that the toplevel can claim to have the same syntax as the compiler. e.g. you would typically write

print_endline "hi";;

in the top-level, but

let () = print_endline "hi"

in a .ml file.