I for myself, when reading papers, almost always miss important information such as justifications for certain design decisions, implementation details or more detailed differentiation to related work.
So for my own papers I tend to generously include such information. However, in the opinion of my co-authors and PhD-advisor, this leads to "too long" papers that reviewers may be unwilling to read due to time constraints leaving a bad impression on the reviewers, and they therefore may be biased against the paper.
If I were a reviewer, however, I would be thankful for every additional information & explanation (even if redundant or maybe "obvious") and would be more annoyed by missing content.
So my question is for those of you that are often involved in reviews, how they think about this issue. Are you, bluntly put, more annoyed by missing details or by "too long" papers ?
EDIT: I think the question is particularly relevant, considering the fact that restriction on the number of physically printed pages is much less relevant nowadays where most information is published and read online.