Ok, off the top of my head....
Produce your ballet flats in standard width too, not just wide fitting.
Stop skipping out random age sizes in school uniforms. Example, girls cardigans go from 3-4, 4-5, 5-6 but then no 6-7 instead you go straight to 7-8. Then no 8-9, but straight to 9-10. My poor daughters end up in cardies with short arms at aged 6/7 because they are too big for the 5-6 but the 7-8s are too big. Nuts.
The kids school shirts with Velcro instead of a top button. Fine for a while but the velarised doesn't last for long and never sticks. Sort it out.
Let us return clothes items to food shops. If I can pick up from there, then let me return it there too. Getting to a 'proper' store is a real pain.
Sort out the location of things, in the clothes areas and also in the food hall. I am stick of traipsing up and down aisles looking for something particular. I'm sure you do it on purpose in the food hall but I've given up so many times and gone elsewhere that it's been your loss.
As many others have said, produce good quality staples in key colours. Think uniiqlo. Stop overthinking details, we don't want them.
Beauty - do not change a thing here please!!!
Changing rooms - I do not need someone to take my items and walk me to a room, funnily enough I can do this myself. Instead, be useful and get me alternative sizes if I need them.
Sort out your website. The photos are awful. And make the stock numbers easier to look up. I don't get the vast array of numbers and letters that sometimes work and sometimes don't. The amount of times I have seen something in store only for you not to have the colour or size I want. I try to find it online and it's impossible. The name of the item often doesn't work online or the stock ref doesn't exist.
I get that you must have different manufacturers for the same styles (trousers in particular) but the labelling, sizing and material is all to often inconsistent. I have actually given up on trousers from you for many years now. Far too confusing. Sort it out.
I'm sure there is loads more but it doesn't appear M&S are listening so what is the actual point other than then a cathartic rant?!