Flip-flops are so comfortable —they're almost like going barefoot. When toes have to grip to keep the shoe on, it causes tiny tears in the ligaments of the arch (a.k.a. plantar fasciitis). Hoorfar blames trendy thongs for the many cases of arch pain that walked through her door last summer.
Long, pointy toes are bad for you. Since the points are purely decorative—your feet don't reach that far into the shoe—they're no worse than round toes. The spot to beware the big squeeze is right below the toes, where your foot is widest and bunions develop.
Flats are easy on your feet. Not if they have no lift at all. A half- or one-inch heel gives necessary support (especially for flat feet with a tendency to roll inward) and prevents calf muscles from becoming overstretched and weak.
Shoes always feel tight at first —they'll stretch. Don't buy shoes that hurt, period. Even a professional stretching probably won't turn an uncomfortable pair into walking-on-air. (You can also try breaking in the shoes by wearing them around the house with thin socks for a few hours daily, but don't get your hopes up.)
To save suitcase space, travel with only one pair. Never wear the same shoes two days in a row, much less for an entire trip: It puts repeated pressure on identical areas of the foot, causing irritation and letting misalignment problems get entrenched.