Prioritization is one of the most complicated, contentious, and crucial aspects of building a startup. There’s always an order of magnitude more tasks that your team can take on. Making sure that your team is working on the highest value work items is vital.
HBR (Harvard Business Review) just released a phenomenal article on prioritization. I’ll highlight the key points here and analyze them by pulling from my startup experience.

The 3 Interdependent Variables in Prioritization
"There are three interdependent variables that are essential for executing any initiative — objectives, resources, and timing. You can’t produce the desired effect of a project without precise objectives, ample resources, and a reasonable time frame. If you push or pull on one leg of this triangle, you must adjust the others. All three variables are important, but resources reign supreme"
This makes sense of course, but is very easy to forget. By speeding up a project, you’ll pull on more resources and clearly alter the importance of certain objectives. If you shift objectives, you’ll shorten the timing for some initiatives, but increase the timing for others (while adjusting your allocation of resources). If you hire or fire employees, the timing of your projects and the importance of objectives will necessarily change. To engineers, this will likely remind you of the CAP Theorem.
The reason why resources “reign supreme" is because they are the leg of the three legged stool that actually gets things done. Without having dedicated resources, objectives and timing just don’t matter.
This is especially true in startups because the team is tiny. A few crucial team members leave the company, and management has to quickly shift priorities and timing of projects. Similarly, making the hiring plan more aggressive causes very obvious and immediate changes to what the team can get done in a quarter.

The 3 Levels of Priority: Critical, Important, and Desirable
"A critical priority is an objective that must be successfully accomplished within a specified amount of time, no matter what."
The “no matter what" bit is the key part. Critical projects should be given unlimited resources towards achieving their goal. The manager running the project is allowed to pull resources (people and money) at will to get the project done because the timing is fixed. These are the classic deadline heavy projects.
"An important priority, on the other hand, is an effort that can have a significant positive impact on performance. For these initiatives, resources are fixed and the variable is either time or the objective"
Important objectives are still objectives that have to get done, but the crucial part is that there is no strict time or objective pressure.
"A desirable priority is an effort in which resources and time are both variables. The organization desires an outcome but cannot absolutely commit specific resources over any specifiable time period."
Desirable objectives would go under the “nice to have" category. These could be optimizations to the current product, infrastructure improvements, etc.

How Resources are Affected
It becomes crystal clear that in this 3 tiered priority system, managers must be very careful about assigning tasks as critical. Critical projects can take on unbounded resources. They can’t pull resources from important projects; those still need to get done. The only option is for critical projects to take on resources from desirable projects.
At a startup, it’s very easy to throw around the word “critical" when we actually mean “important". The word critical is very clear to employees, and it can start to become a catch-all for anything that is necessary to get done. Employees feel urgency to get things done when they’re working on critical projects. But, having too many critical projects causes confusion. Certain projects that are actually important will try to poach extra resources, limiting the pool for truly critical initiatives.
Additionally, the organization can start to feel what I call “critical burnout". If everything is critical, then nothing is. We become desensitized to what working on a critical task really means. This is a disaster. There will actually be critical things to get done, and it’ll be harder to get your team to mobilize if it feels like just another work item.

Thanks for reading! I consider myself a prioritization junkie, and I’d love to hear about your experiences with this subject and how it relates to your organization.