I tend to always just slip my feet into flip flop at anytime of the year. I've been seen in them while its been snowing and I must admit I am far happier in them than I am shoes or trainers. I'm always hot (Asperger's makes me hyper sensitive to temperatures) so I naturally want to wear them.
The flatness of flip flops and flat shoes can stretch the calf muscles too much if the wearer is used to wearing a raised heel. It strains the achilles tendon and the back of the leg but isn't something which really effects me as I'm used to being barefoot or in flat shoes.
One thing that does give me a problem is that wearing flip flops all the time leads to scrunching your toes up into a claw to keep them in place. This can lead to problems such as tendonitis which I suffered from while in America in the weeks prior to my rupture.
I'll post more about how I plan to overcome this problem later.