One of my favorite series is Nathan Lowell's Solar Clipper... in In Ashes Born, there's an bit about creating a logo for the company...

    He pointed to the far end of his studio. Two tiny patches of white—which were probably actually gray—lay in a single pool of light. One was a smudge of red and the other was a spiral of red. “Which one of those is your logo?” he asked. 
    “Neither,” Pip said.
    “The smudge,” I said understanding where the kid was taking us.
    “Right,” he said. “The smudge.”
    “What?” Pip asked.
    The kid held up the paper from the workbench. “Look, this is nice and all, but it’s too fussy. If you look at anybody else’s logo, it’s not fussy. It’s iconic. A crown with wings. A C in a circle. That’s yours,” he said to Pip. “All of them are simple shapes combined to form an unmistakable pattern.”

The goal of an icon is to be able to identify it quickly without having to read the associated text.

The inkwell and the two with the paper are artistic - but they aren't things that stand out quickly when you're trying to find them in the launchpad or on the sidebar.

Pages is orange. Numbers is green. iTunes is red. Keynote is blue.

For Microsoft, Word is blue, Excel is green, and Powerpoint is orange (and Outlook has an envelope like shape). The letter reinforces the choice, but that's more of a hint and reinforcement.

The shape and color is the important thing for quickly finding what you're looking for.