I have to admit that I'm not too keen on the idea of a broad-based "income tax" on players, if only because the game's current mechanics make it all but certain that the main beneficiaries will be people who either can't (due to class/spec) or won't put much gold into the guild coffers. Moreover, the taxation idea acts as an incentive for people not to guild their alts, thus avoiding taxation entirely on toons that are usually the real means of support for a raiding main (someone remind me to go reserve a hunter named Swissbank). As an herbalist/alchemist, I farm a lot for friends and have been known to chuck the guild bank a few hundred gold from time to time. Maybe I'd save time and money under a system that required me to hand over 2-3% of my income, but still. Being taxed removes an element of individual responsibility, and it certainly takes away the nice feeling you have for voluntarily helping others.
If nothing else the idea's given rise to a few nice jokes (Cacora of Hellscream: "Do I get money back at the end of the year if I claim multiple alts as dependents?"), but the final word may well belong to Grig from Whisperwind: "So, Blizzard is considering taking one of the most universally loathed concepts from real life and adding it to a game. Why, they'd be silly not to do it."