Things I make a special trip to M&S to get, despite bitter experience and knowing it'll turn into a wild goose chase:
-Decent quality plain cotton bikini briefs. Except the thickness of the fabric is now half that it used to be and it's easy to put your finger through them when pulling up. The cut is also skimpier than it was five years ago - I'm comparing with old pants I have. M&S used to be thought as the flagship place to go for quality underwear but it certainly doesn't deliver any more.
-tights. I like the thick marled cotton ones for winter, and also their woollen tights. But I very rarely find the magic combination of right size/style/colour. It's maddening.
-school uniform items. Hard to find the right size of trousers you need, presumably because demand is so high.
-Men's trousers. DH is happy with their classic-but-not-Old-Fart styles, like moleskin. Can usually find what he wants. Men's traditional pjs also good quality still, also their woollen socks. Men's stuff in general seems not to have suffered the attrition in quality that the women's essentials have.
-Good quality, thicker denim straight legged jeans. For preference, Roma. Except ... I can't remember the last time I was in an M&S and managed to find that golden combination of correct size, length, style and dye colour. 
Stuff I really hate about going into an M&S:
-the cluttered layout. Agree with the posters who say it's madness to have basics, eg a black skirt, in half a dozen different locations in such a large store. If you came in to look for a basic staple you could easily spend 45 minutes trawling before you were happy you'd seen all options.
-DENIM ZONE. It's a fucking war zone. Heaps of jeans piled up on the floor or slung over rails, no tidying by staff, and worst of all, jeans apparently randomly placed on hangers, so the hanger size bears no resemblance to the label inside. It's infuriating to think you've stuck lucky in your size, only to check the label and see you have an armful of random sizes and lengths.
-Per Una. Just, no. Maybe back in 90's when I was a student there were a few things that were ok. Tops mostly. I like to play the Per Una game in charity shops actually. See how far away I can be and still be able to pick out the Per Una piece on the rail. Extraneous slabs of luridly dyed lace, excess frills or diagonal skirt panels are a dead giveaway. I can't believe they think they have a USP here, or that a team gets paid to design this crap.
-Clutter and scale of most stores. You have to trawl all over to find all the options of, say, a pair of plain trousers, or a black t shirt. Too much stuff spread over too large a space.
-bra fitting 'service' - completely inadequate. Before I stopped using them, I went to some decent underwear places like Rigby and Peller and Bravissimo, and went down two back sizes and up two cup sizes. Have never been back - at least, not to be fitted. M&S bras are not usually to my taste now, though I like their non-wired crop top styles.
-why do they stock a tiny and pointless range of toys, ffs? Or a handful of books? Clearly they're trying to be a one-stop shop for gifts but they can't do everything.