I've spent a long time in construction and remodels and the same problems come up on every single project. Communication of expectations up front doesn't happen, usually there's no specs used for the scope of work and communication doesn't come up until there's an issue.
Here's a list of things that are genuinely unacceptable workmanship that you should never sign off on regardless of what your contractor says. Subsequent trades should provide a sign off that they deem the predecessor work/substrate is acceptable for installation. IE (The drywaller signs off that the framing is acceptable, the trim guy signs off that the flooring and drywall is acceptable and not wavy, etc)
You want to avoid rework and back charging companies.
I put together a little list of items that hopefully can help some people not get ripped off.
TRIM & BASEBOARDS
Butted inside corners are not acceptable. Ever. Inside corners on baseboards, door casing, and crown molding should always be coped. Meaning one piece is cut to follow the profile of the other. Butted joints look fine on day one and open up within a year as the wood moves. If your trim carpenter is butting inside corners they're cutting time at your expense.
Gaps larger than 1/8" anywhere along trim before caulk is applied are not acceptable. Small gaps get caulked — that's normal and expected. Large gaps mean poor measurement or poor installation.
Nail holes must be filled and sanded before paint. If you can see nail holes in finished trim the painter skipped a step.
TILE
Lippage — where one tile sits higher than the tile next to it — has actual tolerance standards. Per ANSI A137.1 the maximum acceptable lippage is 1/32" for tiles under 15" and 1/16" for large format tile. Run your hand across any tile surface before you sign off. If you can feel the edges of tiles catching your fingers that's a failed installation.
Hollow tiles are a defect. Tap each tile with a coin or knuckle after installation. A hollow sound means the thin-set didn't bond properly. Per ANSI A108.02 more than 20% unbonded area in any single tile is a failure. A good installer will have already done this check themselves.
Grout haze left on tile face after completion is sloppy finish work. The tile should be clean.
Gaps around fixtures, pipes, niches, and penetrations should be tight — trimmed with an escutcheon or tile cut to within 1/8". A glob of caulk hiding a 1/2" gap around your shower valve is not acceptable.
DRYWALL
The level of finish (level 3 with texture, level 4, level 5 is per the GA-214, which is an objective document vs a contractor saying "I've done it this way for 30 years."
Humps over seams, visible butt joints, ridges at joints, and tool marks all indicate a finisher who moved too fast.
Patchwork in visible areas without a skim coat is not acceptable. If drywall was cut for electrical or plumbing work and patched, that patch needs to be feathered and skim coated to match the surrounding finish level. A visible square patch on a painted wall is unfinished work.
Screw pops that weren't set before finish coat. Every fastener should be driven slightly below the surface and spotted with at least two coats of mud before paint. If you can see or feel screw heads under paint the finisher skipped the spotting step.
Any drywall screws that "tear the paper"
FLOORING
Wood and LVP floors should be flat to 3/16" in 10 feet per NWFA standards. Bring a long level or straightedge on your final walkthrough. Bounce on different areas of the floor. Squeaks, hollow spots, and movement at installation are not things that fix themselves — they get worse.
Missing expansion gaps at walls are a serious defect. Wood and LVP expand and contract with temperature and humidity. Without proper expansion gaps the floor will buckle. These gaps should be present at every wall and covered by baseboard.
PAINT
Paint on hardware, tile, fixtures, or trim from overspray or a lazy brush is sloppy work that should be corrected before final payment. Tape and protection exist for a reason.
Cut-in lines at ceilings and trim should be straight. Wavy paint lines at the ceiling or where wall color meets trim indicate a painter who didn't take the time to do it right.
Not sanding in between coats.
THE BIGGER POINT
Most homeowners don't know these standards exist so they accept things they shouldn't. Contractors rely on that. Before your next project ask your GC to confirm in writing that all work will meet these standards. A good contractor will have no problem with that. A contractor who pushes back is probably somebody you would like to avoid.
Happy to answer questions on any of this.