The first pass provides you with a high-level view of the contents. Helps you avoid getting stuck in a badly written, uninteresting, wrong or simply beyond your current knowledge text. To get the most from the first pass I usually start with references. And although statistically 80% of the authors never read the cited texts in full , they provide a great overview of what to expect. Whenever approaching a complex, long paper I tend to mark the cited papers I've read. So I can get back to my infamous notes) or verify the background. Reading the papers starts with the title, abstract and introduction which I read carefully the first time I approach a paper. They are the first sign of whether the article is the one to go with. After that I scan through the article focusing on section headings, graphical elements and math formulas. I also read the results and discussion section to have an overview of where I will be lead with the article. The author of "How to read papers" paper, that was recently trending over at HackerNews suggests that the first pass shall give you an answer to 5Cs:

Category - what type of paper is this?

- what type of paper is this? Context - which other papers it is related to? which theoretical bases were used to analyze the problem?

- which other papers it is related to? which theoretical bases were used to analyze the problem? Correctness - do the assumptions appear to be valid?

- do the assumptions appear to be valid? Contributions - what are the paper's main contributions?

- what are the paper's main contributions? Clarity - is it well written?

For me after the first reading, aside from the bird's eye view of the paper, allows to decide whether I should give it a second pass. Personally I ask myself:

What's in it for me?

Do I have enough background?