Likelihood of acceptance: Do your subject, approach and results fit the mandate of the journal (is yours the kind of manuscript they’re looking for)?
Prestige for your CV: How good is the journal's reputation? How high is its impact factor?
Prestige for journalists: Are papers from this journal often reported in the mainstream media?
Readership: Does the journal cover a specialized topic or a broad area of science? Which kind of audience are you writing for?
Ease of finding for readers: Is the journal indexed by everything? How well can you use keywords in the title and abstract to bring in readers from Google Scholar and other search engines?
Cost of publishing: Are there page charges? Optional or required publication charges for open access? Charges for colour figures (only an issue for print journals)?
Online supplementary materials: Does the journal host these? What are the limitations?
Copyright and licensing: Must you sign away your rights? Can others reuse your material (e.g. use your figures in teaching)?
Turnaround time: Rapid pre-screening? Total time from submission to publication? Online early access?
Is this an ethical publisher? Elsevier? Other for-profit? Society journal? Predatory publisher (see Beall's list)?
Type of publication: Online-only? Print edition only? Both?
Tiresomeness of Instructions to Authors: Will getting your figures into the obscure required format force you to spend $500 on the full version of Photoshop?
Might the journal highlight your paper? Does it include a News and Views or other section that highlights some papers in each issue?