In no particular order, apart from no 1 which has has held its pride of place for some time now:
1. Shower curtains, useless, never attractive no matter what silly design they have, fungus magnets
2. Black and White schemes, never elegant, black and cream, black and stone at a push but never ever black and white and beware of too much black furniture.
3. Brilliant white walls, unless you want the mental asylum look, white should always be 'off' no matter how slighlty
4. Lever handles, especially modern ones:
The eye focuses on details such as light switches and door knobs and immediately bring down the quality of a build if chosen and positioned carelessly. I once, years ago working as an assistant, forgot to specify a deeper lock so as not to graze the knuckles when turning the knob. All the doors had to be refinished days away from handover.
5. Thoughtless electrical plans. The electrical plan should be put together by the decorator together with the furniture layout, one cannot be determined by the other. You need twice as many plug sockets as you think you do, and 2 or 3 way lighting circuits, dimmer and lamp circuits make for a relaxed atmosphere and easier living.
6. Granite worktops. They just look cheap and common and you never know how clean they are since they hide so much of the dirt, which I think someone somewhere thought was a clever idea back in the late 90's. There are countless beautiful stones of varying degrees of practicality which have far more character, including the new reconstituted quartz stones for convenience.
7. Primary colours, not even in a nursery! Colours should always be a little mixed and unclear.
8. Outdated lampshade shapes, the quickest way to date even the most stylish interior.
9. Modern - as in recently produced - crystal chandeliers
10. Bad kitchen layouts put together by people who neither cook nor clean! No, it is not just a matter of the 'triangle' between the cooker, fridge and sink.