flexible: Bugs doesn't decide whether you should think in term of priorities, types of issues, milestones, etc... Instead, Bugs uses a Tags system that you can set up to match your own way of dealing with stuff.
open source: was, is, and always will be.
Multilingual: so your clients can feel at home with it.
Handy: issues can be written in Markdown, commented so you keep the discussion organized, and have print-screens.
Reliable: you get a warning email whenever someone comments, closes or assigns you a bug ticket (unless you did it to yourself, you sick mind).
Default Assignee per project
Several roles & privileges:
User: can only read the issues in the projects they are assigned to
Developer: can update issues in the projects they are assigned to
Manager: can update issues in all projects, even if they aren't assigned
Administrator: can update all issues in all projects, create users and view administration
Create a MySQL Database
Make /uploads/ write-able (CHMOD 777)
Open /install/ in your browser
Delete or rename /install/
Tested on: Apache, IIS
PDO Extension for PHP (MySQL)
MCrypt Extension for PHP