For most business analyst, our requirements management software pretty much stops at Microsoft Word, Excel, and Powerpoint. Some of you fancy folks might also use Visio. While, It is entirely possible to do you job as a business analyst with these tools, it usually means you are also having to keep several files in sync, which can become tedious. This basically means a huge chunk of your job is document management instead of business analysis. Business analysis software should help you take that tediousness away, so you can focus on the job you are actually paid to do. Now then, lets get to the features.
First, Ease of Authoring
This is a big one and it covers quite a few things, but it boils down to, you need the flexibility to put in whatever you want, how you want it. Rich text editors are a must for business analyst software, because it allows you to create easier to understand text based requirements. Also, the ability to add additional attributes to requirements, like priority, type, category, etc that make it easier to organize your requirements. This is typically where Microsoft words comes short and you need to keep a separate excel file to organize and categorize. Now you have two documents to keep in sync.
Traceability is extremely tedious without business analysis software or tools. Traceability is really important to make relate different items with each other. Some examples are tracing business requirements to solutions requirements, tracing requirements to process tasks, tracing mock-ups to process steps, etc. The list can go on forever, but traceability is important because you can easily understand when making a change to something, all the potential places it can impact. That's huge and will save you a lot of headache in future changes (*ehem* defects *ehem*)
None of the other stuff matters if you can't easily share it with you stakeholders and keep track of feedback. Most of us use emailed documents to get comments, feedback, and approval. Meaning you are having to keep track of a bunch of emails. Email get lost and missed all the time. Its better if you can have one place to keeps everything where people can comment and approve so you keep a full history of why you did what. It also helps you have to bring new people into the fold you won't have access to the hundreds of email conversations you've had.