Finished Nova Covert Ops on Brutal. My-my... This is probably the best Blizzard RTS campaign I've played. I feel like they've been improving gradually with every new campaign, and the gameplay really shined in this one.
The story was a bit naive, and a bit incomplete IMO (what happened to Alarak and Stone?) but everything else was stellar. A decent amount of challenge as well; the last mission was something to behold in terms of challenge, and I probably broke my F10 button, reloading constantly. I liked it how you could approach most missions from different angles, try different strategies and unit compositions. Unit upgrades this time around were very inventive, as well as some of the Nova's equipment.
I really hope they keep the mission packs coming, there is a lot of potential here. As of now, I'm even considering buying commander Nova for coop (even not being a big fan of paying for this kind of content), as I really liked the gameplay in the campaign.
All I can say is, it is a matter of experience. For me the checklist is probably hatcheries-queens-supply-army, with creep spread added when I'm not forgetting it. When to drone and when to make units is a very complex matter, and it really depends on the situation. Generally, Zerg is much more mobile than most other races, due to fast unit movement speeds, low hatchery cost, ability to produce multiple units at the same time per 1 hatchery, overlords and overseers used for scouting and some other things, so you usually need to play more aggressive than other races to really utilize racial advantages. I think Zerg is the hardest race for the beginners, and possibly the hardest race for the pros, but the easiest race for somewhat-above-average players, because of how simple macro is compared to other races, and micro being less intensive than that of Terran. Plus, the ability to withstand cheesy rushes in 4v4 games, which is very important in my book!