Hating a 'drivetrain' choice on one bike vs. another is as imbecile as people getting into fist-fights at a motor race that they are not actually racing in Australia over a debate between two car companies. It really is ridiculous.

Now, shoes again. Specifics and details.
Ninety-nine percent of the riding I've done over the past 5 years has been in Teva sandals. I use the Hurricane model, the cheapest, flimsiest, lightest model Teva makes. They cost $39 and weigh 10 ounces in a size ten. In the summer I wear light socks or none; in the winter I often double-up thick wool socks. Being sandals with adjustable straps, there's never a fit problem, there's never the feeling of crampedness or anything.
But what's the lower limit, shoeswise?
Proabably Crocs. Several people we know ride in them and prefer them to just about anything else. Before you think how ridiculous Crocs are, or how unsuitable for pedaling, consider that they're just contoured foam with enough resistance to offer support. They are not high shoe-craft, but with the right pedals beneath them, they do just fine for lots of riding, including long distance touring (we know people who prefer them to all others for just this purpose).
I recently got some fake Crocs, from Target. They cost $9.99 and weigh about 5 ounces a pair. My foot fits in them loosely with a wool sock, and when I flip the heelstrap out behind my heel, it doesn't even do any good, because my heel doesn't contact it.
I've been riding them lately and the grip is excellent, even better than Tevas. I timed a hillclimb that I do all the time, and my times are no different than with Tevas (or real cycling shoes, for that matter). The fit is loose, but it doesn't seem to matter. The comfort is out of this world. They work great with our Sneaker pedals, and on any ride that didn't involve dragging the bike up steep, loose trails (hiking with it), they're perfect.