Luck has several merits as a design feature.
- It builds upon and extends the existing mechanic of account-wide bonuses from the achievement system, which is good economy in design.
- I am generally in favor of making bonuses like this account-wide rather than character-specific so that players are not locked into one character. There is also no perceived need to switch to a particular character or suit of armor before using the mystic forge.
- Luck exhibits further economy in design by making it a gold sink.
- That gold sink creates an investment question: what is your break-even point on how many silver pieces you will give up now to get better loot then? Given the high cap on luck, this is an effectively bottomless gold sink for most players.
- As a designer, this seems like a good way to reward long-term players and encourage players to stay for the long term. You have the chance to sink some silver now to earn more silver for the rest of the time that you are playing. That pays off big if you are a multi-year player; it potentially takes away from short-term players if the loot tables are built around an assumption of +200% magic find, but they were leaving anyway, and they are not likely to notice a small failure to gain (and most of the failure to gain is in seeking rare endgame items that short-term players will never be pursuing). And if short-term players salvage their items instead of contributing to inflation? All the better for the economy.
But what I really like is that players no longer have the option of getting more reward for contributing less. Now they have the option of potentially getting more reward in the long run for definitely receiving less in the short run. You pay the cost for your benefit rather than imposing it upon others. That is good design.