A “good” trick/combo has to have a certain amount of character, indepthness, flash, appeal and structure.
Tricks are usually in the range of few steps with the main element expressed strongly. Think of the trick “Boingy Boingy”. We have the split bottom mount which then goes into the “hop” or “boingy” effect on the string. The main concept is shown clearly.
Combos on the other hand are a bit difficult to judge on if its “good” or not. One can judge a combo from a artistic point of view and a competition point of view. Back in the day of responsive yoyos, combos didn’t last as long, from 10-20-30 seconds. Now with unresponsive yoyos combos can last near a minute or longer!
From an artistic view, you need to see how it represents itself, how it “feelts”. Good use of elements within the combo is always a plus. Is it graceful or raw? Yoyoing is subjective due to the creativity so someone may like something but someone else definitely will not. Does the combo have good structure? Does it flow well? is movement continuous, no jerkyness. Is there “depth” for it where there is layers of technical execution? Is it innovative or original? See these little things all matter.
From a competition point of view, people would look at it like above but also make mention about “does it score points?” Is the combo using space effectively to get max amount of points for space use? do you have enough speed to make it look good? Such examples like this would be it for a trick or combo.
Yoyoing is art, a sport and a hobby all rolled in one. In my best opinion, there is no definitive “good tricks” because when I watch people yoyo I like it all. What I said above is only just cutting the surface, there is a lot to it to determine if a trick or combo is “Good”.
Just my two pennies,