The most common complaint is noise - a grinding or popping sound as the door travels, usually worse on one side than the other because rollers rarely wear evenly. After that, it's binding: the door hesitates or jumps at the same spot in the track every time, which is a roller flat-spotting or a bearing seizing inside the wheel. On the 1980s-era doors common here, with their steel 7-ball rollers, you'll often feel it in the opener before you hear it - the motor straining harder than it should for a door that isn't otherwise heavy.
Less common but worth mentioning: a roller that's worn enough can pop fully out of the track, which drops that corner of the door and can crack the panel if the door keeps trying to run. If you hear a new sound and then silence a second later, don't keep hitting the opener button - that's usually the moment to call rather than the moment to test it again.