designing logo is a great hobby... i personally like to design logo, even nobody requested them.. hehehe... sumetimes it takes practice, n research to came up with a good logo design..

for me, the criteria of a good logo are divided as follows;

a) Typo -- If a logo include typography, than it must be parallel with the overall design n concept.. avoid hard to read font.
b) Color -- Choose less than 3 colors, 4 is okey, but do avoid.. if u dont know how to select the color, ask ur clients, do they have any preferred color, or corporate color?.. make sure that the color are easily adaptable, think about how the logo will be use.. some colors look great on white background, but its not always look good on black/dark bg... so plan ur color wisely.. u can go to colourlover.com for inspiration.. if possible, present ur logo mockup with black&white color as well.. sometimes, logo doesnt look good when photostated or printed in B&W
c) Strength -- a logo must be strong, bold, and precisely defined what it resembles.. try to use elements that closely relate to the function of a logo.. if it is for a gardening company, try to use flower, leave or etc.. u get what i mean right..
d) Composition -- if ur logo comprises of typo n image, make sure to align them the best way. present ur client with multiple positioning example, so they have more choices.
e) Creativity -- lastly, a logo must be creative, unique... this may be the hardest part.. but if u practice alot n do some research... u will get better.. hehehe...


The most important thing is that u have to follow what the customer wants.. sometimes they will choice the worst logo.. but u cannot force them to go for the one that they dont likes, even if it looks great.. always discuss wif ur customer n always be prepare.. hehehe ..