I've added my comments on the replay at the bottom, but you can also find them on youtube.

So I think at the core of your problem is the question of: Why do I expand? and I can understand your frusteration. A lot of people will say: "Zerg should always fast expand," and if you take them at their word you might not understand whyearly expansions are so helpful to Zerg.

Powering Drones

There are a number of times when you want to power drones:

Drones vs Army

When attacking

Spare supply

Out of gas

Unlike their Terran and Protoss equivilents the Zerg can produce Dones at a much faster rate, the trade off being that they have to choose between economy and army. While the other races have to make this choice as well in mineral distribution, Zerg also have to make it in production. So Zerg players often alternate between powering drones and building units.

Ideally you'd like to hold off on building units as long as possible and then only produce them right before your opponent reaches your base. This isn't usually practical, but when discussing when to power drones it is useful. Unless you need units you want to be building drones and upping your enconomy. So the question of "When to power drones" is best answered by knowing the size of your opponents army. If you have an army close to your opponent's in size: Power Drones. Otherwise, you might want to consider building up your army or teching some counters to his army.

A really non-intuitive time to power Drones is while you're attacking. While you usually want to reinforce an attack at first, additional reinforcements will often arrive to late to be benefitial and are actually a waste (especially if they get caught). So if you're in the middle of a large assault and you've already built a wave of reinforcments, now may be a good time to power Drones as its unlikely your opponent will counter until he has handled your assault.

A more obvious time is when you have spare supply. Since most Zerg units take 2 or more supply, If you ever find yourself 1 supply short of cap, its a good idea to build a drone to utilize this supply (after you start an overlord of course).

Some Zerg strategies call for not-stop production of units: ie Mutalisks, in this case you want to build drones when ever you run out of gas. While you will want to build some overlords and Zerglings, you don't want to ever be in a situation where you have gas, but no minerals. To this end you need to build up more drones so that you can help saturate your expansions when you do expand.

Expanding

So the purpose of expanding is twofold:

Provide more larva

Provide better mining

Back in Broodwars we only had as much larva as we had Hatcheries, so it was common place to get a large number of expansiosn largely for the production purposes. Since the advent of the Queen, this has become less necessary. A Queen provides the larva of 1.5 expansions. As a result if you have a Queen in your main, your expansion is only increasing larval production by 40%. By comparison, if you have an expo + main and no queens, then the queen is increasing your larval production by 75% (150% if you get two queens). So the key thing here, is that its the queen that really increases larval production, and if you're expanding and don't have one you're kind of missing out.

It was discussed how there is a soft cap on miners at about 16 (2 per patch) and a hard cap at 24. As a result expanding gives you more places to mine from, and even an opportunity to boost those extra 8. On the other hand if you are under 16 miners, expanding isn't doing much for you. As a result, expansions are largely a function of how many drones you have. If you're powering drones properly you usually find there is no time you don't want an expansion. However, if you're short on drones (for harassment or whatever reason) expanding isn't netting you much, so you should reconsider.

The statement: "Zerg should always FE," rises from early game Drone powering by Zerg players. If you find yourself under early pressure and you're not able to utilize that expansion, it largely was a waste. Sometimes, its important to ask yourself "why do I want to expand?" instead of "can I expand?"

So here were the specific things I pointed out for the replay: