Many people have observed that the new iPhone 6/6+ feels significantly more slippery compared to the last few iterations. It's in fact as slippery as the original iPhone and the following test demonstrates why.
Rest the iPhone flat on your fingers (palm out, fingers together) and shake your hand back and forth sideways.
You'll notice that the iPhone 6/6+ slides off quite easily whereas the previous generations (except the original iPhone) don't slip very much or at all using the same movements. That's because the glass back (on the 4/4S and, to a lesser extent, on the 5/5s) and the plastic back (on the 3G/3GS) provide significantly more friction against the skin compared to full aluminum back of the iPhone 6/6+ and original iPhone. The rectangular design of the iPhone 5/5s also helps in sliding less compared to iPhone 6/6+'s rounded design.
P.S. I'm not responsible if your iPhone falls off and shatters while performing this test
Rest the iPhone flat on your fingers (palm out, fingers together) and shake your hand back and forth sideways.
You'll notice that the iPhone 6/6+ slides off quite easily whereas the previous generations (except the original iPhone) don't slip very much or at all using the same movements. That's because the glass back (on the 4/4S and, to a lesser extent, on the 5/5s) and the plastic back (on the 3G/3GS) provide significantly more friction against the skin compared to full aluminum back of the iPhone 6/6+ and original iPhone. The rectangular design of the iPhone 5/5s also helps in sliding less compared to iPhone 6/6+'s rounded design.
P.S. I'm not responsible if your iPhone falls off and shatters while performing this test