I think the whole critical outcome makes the game more interesting to watch. Its like a slot machine but I feel it could be made even more insanely awesome.
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[1.] I feel every skill should have some form of critical success, not just harvesting.
[2.] Numerous types of critical successes. Such as rare gems from mining, more gold from thieving,
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I feel alchemy would be insane in this game!
[1.] Crafts buff potions that last 5 minutes like bonus XP, bonus luck.Bonus combat stats,, etc etc..(Non subscriber skill for many reasons. One being that if it was free, it would create a huge market for both free and pay users in the auction house. Tons of trading...)

I like these ideas. For expansion on your critical success how about:
Agility: You perform a trick while doing the action and spectators throw tokens at you (double tokens)
Cooking: The [meat/fish] you put on the skillet was so large you get two servings from it
Fletching/Smithing/Tailoring/Leatherworking: Your skill in [skill] is so great you manage to save some of your materials ie: spend 38 cowhides to make the chestpiece rather than the full 40
Jewelcrafting: I don't think jewelcrafting should crit.
Prayer: The dark lord Cthulhu hears your prayers and you receive a boost to x skill for 2-3 minutes
Runecrafting: The runestone/runes you used seems to have been more pure than usual. You receive twice as many runes
Thieving: You see an opportunity to grab more loot and do so. Twice as much loot

These are just some examples. Could be expanded to include quite a bit more stuff.