Where the gloss came from
The accepted answer credits the influence of one desktop interface introduced at the turn of the century, quoting its designer's stated goal that people should want to lick it when they saw it. That look was enormously copied, and the thread treats it as the origin of the reflective, rounded, deeply shaded style that followed across the industry.
The functional argument for dropping it
- Gloss and gradients were doing a job: signalling that a thing is a control you can press. One answer argued people no longer need that cue, so the reason for it fell away.
- The differentiation problem: once nearly every icon carries a similar sheen, that sheen cannot help anyone identify one from another. A flat icon has to distinguish itself by its actual shape.
- Texture and reflection were also described as adding to what the viewer has to process, for decoration rather than for meaning.
The argument that it is mostly fashion
Several answers treated this as a swing rather than an advance. On that reading, reflective interfaces communicated technological sophistication, and flat ones communicate having moved past the ornament — simplicity and efficiency as a stated value. One answer pointed out that flat design is not new at all, placing it in a line with mid-century international style, and framed the shift as expressing what the industry currently wants to be seen to care about.
What survives the swing
- Whatever the style, a control still has to look like one. Removing the cue entirely is the failure case flat design has to work against.
- Decoration that every element shares stops being information. That holds regardless of which decoration is fashionable.
- This thread is from 2013, at the peak of the change it describes. The pendulum has since moved back somewhat towards depth and shadow, which is itself evidence for the fashion reading.