Normally <backspace> is bound to backward-delete-char-untabify, meaning that if you have the cursor to the right of a tab, and then backspace, it will convert the tab into 8 spaces and remove one. I don't find that intuitive. I'd prefer that backspace delete a single character, whatever it may be.

(global-set-key (kbd "<backspace>") 'backward-delete-char)

I'd also like to be able to tell if a file is indented with tabs or spaces. I like tabs for indentation, and spaces for alignment, personally. Let's enable whitespace-mode, allowing us to see a variety of whitespace-related issues:

(setq whitespace-style '(face trailing tabs empty space-after-tab tab-mark))
(add-hook 'text-mode-hook 'whitespace-mode)
(add-hook 'prog-mode-hook 'whitespace-mode)

Now we will see tabs as ยป, and trailing spaces and lines in the file will appear red.

By default, M-SPC turns a run of whitespace into a single space, and M-\ turns a run of whitespace into nothing. It's more useful to have a single key binding for this that cycles through those options, and back to nothing.

(global-set-key (kbd "M-SPC") 'cycle-spacing)