Being myself a UI/UX designer,"we get deeply involved in the product buildup to a point that an emoticon like thing can often seem implicit. Why in the world would someone not get it? It’s so implicit to us that it’s never even a point of discussion or question." I couldn't agree more and have personally experienced a lot of times. So I have made a habit of testing everything with friends and first time users. It really effects and help improve UX a LOT.

User testing should really be focused more.

For the case of icons and text.I really feel that the situation and context is important. Like for road sign I strongly feel using combination of icon and text really works the best.