There are many ways to play Nova... this one lets you lane clear, take bases faster, and be stupid deadly due to low cd's level 16+... you become more of a team player due to cd's rather than a shoot and watch teammate.

If you have an Abathur that goes ultimate evolution (hero clone ult), then I highly advise the snipethrough cd nova build... with rewind nova herself nukes... imagine the damage if you get any opponents lined up... you could instantly take 2 or more of them out.

Abathur Build (warning, wall of text and ramblings):
http://www.heroesfire.com/hots/guide/abathermite-wall-of-text-w-ramblings-clone-locust-strategy-5261

(Not embedded for a reason (some peoples browsers might not support embedded links)).

This is arguably a later game Nova... and whilst I used to go envenom... the AP buffed at level 4 is phenomenal, albeit your early level gonks will be weaker... decide which 1 at your own discretion (gain some early game power and lose some late game power, or vice versa).

The ult is situational, but you will be going rewind since the damage of your q is nearly on par with your precision strike... I lean precision strike since opponents tend to be near 1 hit KO with the double snipe at level 20...

Additionally this build allows for Nova to lane clear once your level 16+... which becomes quite useful.

Enemy mercs are a great place to pick a fight since the mercs are harder to kill, it means lower cd snipe shots.