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People has computer problems.
No 3-D style logos. 3-D logos are dated, very 90s. Also they are harder to reproduce in multiple formats (on schwag, in print, etc.). Any truly good logo (IBM, Fedex, Coca-Cola, etc.) holds its own in simple black and white - color is simply cosmetic. 3-D logos rarely meet this standard.
2-color logos are cheaper to print; anything more than 3 colors and you pretty much lock yourself into digital printing which means colors aren't likely to match from item to item. That's another argument for a logo that looks good in black and white.
With symbols, make sure they have a purpose, and while obvious is sorta good, too obvious will be cliche (e.g. cogs in the case of this contest).