For years I judged mugs purely on how funny they were, which is how I ended up with a cupboard full of novelty print jokes and nothing I actually wanted to drink out of at seven in the morning. What fixed it was paying attention to how a mug felt in my hand before I cared what it said on the side. The weight, the way the handle sits, whether the rim is thick or thin against your mouth. That is the whole game, and nobody tells you until you have already bought the bad ones.
A well made mug has a bit of heft without being a dumbbell, a glaze that is smooth and even rather than gritty in patches, and a handle big enough for a proper finger or two, not just a thumb loop that was clearly designed by someone who has never actually held a hot drink. Look at the base too. A good one sits flat and quiet on the counter, a bad one wobbles or has a seam you can feel where the mold didn't quite close right. Stoneware and porcelain hold heat better and survive the microwave without turning into a branding iron, while thin bargain ceramic cracks along hairline fractures after about eight months and then just waits for the worst possible moment to finally give up on you. Where mugs fall short is as anything you actually need to perform, big batches of iced coffee, travel, anything that gets knocked around in a bag. That is what tumblers and travel cups are for, and asking a mug to do that job is like asking a dinner plate to be a frisbee. It will technically fly once.
Scale matters more than people admit. A set of oversized mugs looks generous in the store and then swallows your cupboard shelf, leaving no room for bowls, and a set that is too dainty means you are refilling your coffee at nine, ten, and eleven. I keep a mix now, a few smaller ones for tea and a few larger for mornings that need it, and the shelf still closes. On color, plain white or a soft stoneware glaze in cream, sage, or warm gray wears better than anything with a bold pattern, because patterns date fast and chip in a way that shows, while a solid glaze just looks a little more lived in over time, which honestly suits a mug. The detail that makes a mug worth keeping, though, is a glaze that stays warm to the touch without burning your palm, so you can wrap both hands around it on a cold morning and just hold it there for a second before anyone else in the house needs anything from you.