An oblique positive characteristic.

"Zing" is difficult word to define.

If you have zing - adjective - then you have, perhaps, charisma. Energy. Magnetism. A spark. "Action, animation, ardour, avidity, bounce, brio, briskness, buoyancy, cheerfulness, cheeriness, devotion, dynamism, eagerness, earnestness, ebullience, effervescence, élan, elation, energy, enthusiasm, esprit, excitement, exhilaration, exuberance, feeling, fervour, fierceness, fire, flair, flourish, force, gaiety, gusto, heat, impressiveness, intensity, jazz, jollity, keenness, life, lightness, liveliness, might, oomph, panache, passion, pep, power, sparkle, spirit, spiritedness, sprightliness, strength, style, vehemence, verve, vibrancy, vigour, vim, vitality, vivacity, warmth, weakness, zap, zeal, zest, zip." It's a characteristic, a positive one, especially when you're performing in front of other people, perhaps as a reporter, a dancer, or a gymnast. She's got zing. She's got... I'm all out of synonyms. Zing is good.

Lady: Zing! Zork! Kapowza! Call it what you want, in any language it spells mazuma in the bank!
Lisa: 'Zork'? What is 'zork'?
Lady: I didn't say 'zork'.
~The Simpsons

"Zing" is also an onomatopoeia, usually standing for the noise something small (such as a bee or a tennis ball) makes as it whizzes - zings (verb) - past your ear at extremely high speed. Bees, of course, make a high-pitched noise most of the time in any case, so zing - not being far from buzz - is a pretty good term. Alternatively and most appropriately it is used for the noise a projectile (bullet) makes when ricocheting off something metallic. "PYEEEOW!", maybe, in real life, but in a comic book, you're more likely to read "Zing!"

"Ziing! The extra 'i' stands for extra zing!"
~Pokey The Penguin

Most recently, "zing" has passed into internet slang to become both a noun and a different kind of verb. In an instant messaging, IRC or web forum conversation (and occasionally in real life), a zing is a victory, a funny joke, a moment when one party does something awesome (such as achieving some amazing high score on the Flash flavour-of-the-month), or makes another party look bad (such as turning somebody's ill-thought-out words against them for comic effect). In the latter sense, the second party is said to have been zinged or zing'd (verb), which means roughly the same as "owned", itself carrying approximately the same meaning as "dominated". It is typical to exclaim "Zing!" immediately after a zing (noun) has been successfully executed:

Person 1: Your room smells like ass.
Person 2: That's because you're in it. ZING!
~LT5k, Urban Dictionary

It may or may not be a coincidence that in this context, "Zing" has approximately the same interpretation as "zeng" which, according to Mike Myers' character Wayne Campbell in the movie Wayne's World, is Cantonese for "excellent".

The past tense of "zing" is presumably "zung".