The honest version of "does it integrate with my software." These are the systems this industry actually runs on, and what each one means for a build.
ClioPractice management
The most common system of record in small and mid-size firms — matters, contacts, time, billing and documents. Well documented and straightforward to build against, which makes it the natural anchor for most agents in this industry.
MyCase / PracticePanther / SmokeballPractice management
The main alternatives, with similar shapes. Smokeball is notable for automatic time capture, which changes what a time-related agent needs to do.
NetDocuments / iManageDocument management
Where firms with real document volume keep the file, with the version control and security model that practice management alone does not provide.
Document assemblyDrafting
HotDocs, Lawyaw or template systems built in Word. Where precedent becomes a first draft, and usually the least automated part of a firm that does repeatable work.
Court and docketingDeadlines
Electronic filing systems and docketing services. Deadlines are the highest-consequence dates in the business and are frequently tracked in a calendar maintained by one person.
E-signatureEngagement
Engagement letters, retainer agreements and client authorizations.
Trust accountingFinance
Client trust ledgers with strict rules about what may move and when. Reconciliation here is not optional and is a natural fit for exception-based checking.
Intake and phoneNew business
Where new matters arrive, often after hours, often to a voicemail box, and where a slow response loses the client to the next firm on the list.
Legal researchReference
Westlaw, Lexis or their alternatives. Not an integration target so much as a boundary — an agent that answers from the firm's own files should not be pretending to do research.