I am gonna make some assumptions about your context here, if they are wrong correct me plz and I will adjust my answer.
Marketers has direct access to developers, meaning there isn't a product manager role.
Developers work more like supporting the system. Fixing and developing features that come to mind of marketers.
With these assumptions I will answer the questions:
What are the main and the most important things in the brief for the developer?
I would suggest follow some agile product management approach. With epics and user stories. Below I show an approach with user story.
Context (Story): Information about why do the project is important to create rapport and helps us in technical decisions with some tradeoffs.
Deliverables: What its expected?
Definition of Done (DoD): How do you expect the deliverables to be delivered?
What is the priority?: Here I would use some product prioritization framework. I suggest RICE or Value vs Effort (or both RICE for the bigger scope and Value vs Effort for the lower scopes). The answer to this question has two parts, business and technical. Marketers should answer for the the business and developers for the technical.
Example
Context: As a marketer, I want an a/b landing page, so that I can measure the effectiveness of my two approaches.
Deliverables: Landing Page and Report File
Definition of Done: The landing page must have the possibility to create two versions of the page for a/b test purpose. And click rate of both versions must be delivered to an report file in excel format.
What is the priority?: It has the potential to increase our revenue in 10%.
What is often forgotten to be included in the brief (something needed for further work)?
IMHO, It's that a project brief shouldn't be a set in stone. Generally speaking, a software project is an endeavour, so there isn't a clear path in most of the time, but by the interaction of the group with an open minded point of view its more feasible to achieve the results.
Business rules and restrictions, it's important make explicit what lines we are delimited. This can avoid future misunderstandings and frustrantion.
What is not as important but appears in brief?
All projects/stories prioritized as 'critical'. This shows no sense of what urgency really is. For this, I suggest check for product prioritization frameworks
How can marketers help developers with his/her work? What information to provide, how to set a task?
Avoid interruptions, developing is an intense abstraction exercise and the interruption can cost some hours to the developer return back to the stopped point.
Both marketers and developers should have an open mind avoiding preconcepts to work for the goal of the project.
Only developers set tasks, tasks are the smallest unit in a project and by their nature technical. Marketers should set stories and charge for them, not the tasks.
How do marketers interfere developers while thinking that they are being useful?
Asking for something that was not planned and prioritized.
Just a last comment, software projects are endeavours, and by this I mean its a constant change, it changes by context and time. Creating your article is a great start but putting in practice and then, reviewing your article it will create more value than just a first version.
I am gonna make some assumptions about your context here, if they are wrong correct me plz and I will adjust my answer.
With these assumptions I will answer the questions:
What are the main and the most important things in the brief for the developer?
I would suggest follow some agile product management approach. With epics and user stories. Below I show an approach with user story.
Example
What is often forgotten to be included in the brief (something needed for further work)?
IMHO, It's that a project brief shouldn't be a set in stone. Generally speaking, a software project is an endeavour, so there isn't a clear path in most of the time, but by the interaction of the group with an open minded point of view its more feasible to achieve the results.
Business rules and restrictions, it's important make explicit what lines we are delimited. This can avoid future misunderstandings and frustrantion.
What is not as important but appears in brief?
All projects/stories prioritized as 'critical'. This shows no sense of what urgency really is. For this, I suggest check for product prioritization frameworks
How can marketers help developers with his/her work? What information to provide, how to set a task?
Avoid interruptions, developing is an intense abstraction exercise and the interruption can cost some hours to the developer return back to the stopped point.
Both marketers and developers should have an open mind avoiding preconcepts to work for the goal of the project.
Only developers set tasks, tasks are the smallest unit in a project and by their nature technical. Marketers should set stories and charge for them, not the tasks.
How do marketers interfere developers while thinking that they are being useful?
Asking for something that was not planned and prioritized.
Just a last comment, software projects are endeavours, and by this I mean its a constant change, it changes by context and time. Creating your article is a great start but putting in practice and then, reviewing your article it will create more value than just a first version.
I hope the answers help you =)
Hi Jackson, a helpful answer, thanks!