Hours (surprisingly), pay (... but only when hired directly by the firm or a partner, not through a third party such as a legal recruiter or document review employer), friendliness of 'regular' firm attorneys, flexibility in terms of hours and working conditions (after I did a successful job on the first contract (i.e. gained their trust) the firm allowed me to work remotely with a few minor but reasonable conditions).

Upward mobility / opportunity as an associate on regular partner track (i.e. parlaying a staff or contract position to regular employment) is very low.

The firm doesn't advertise these 'contract' positions on a public forum and even legal recruiters may not know about smaller projects so try to focus on networking and/or building relationships with partners directly and more specifically, the partner who leads a specific department, and then contact and/or offer your services directly and remember to follow up because they may not need you now, but you never know when an opportunity may arise. Also, the litigation department / partners, especially on bigger cases, often 'delegate' legal work to outside firms and/or attorneys so maybe target said 'outside firms' and offer assistance and/or consult with them to try and ascertain other relevant factors--i.e. timing, type of case, delegating partner, etc.