BI projects have complex elements such as dimensional modelling, modelling facts, hierarchies, calculations etc.. This makes it essential that Business Analysis is applied on all projects. It's during the BA phase that requirements are picked up.
As a BI professional it's important you use your judgement to tease out requirements that users may not be aware of, for instance, slowly changing dimensions, late arriving facts, late arriving dimensions.
You can also start to detect data quality, although, data profiling is the key tool to determine data quality. Users will often give their opinion on data quality.
You can also get an understanding of the largest tables and any fact grained dimensions. For some BI projects there are limited facts, we see this with HR & Legal.
It's very important to determine this early on. This doesn't prevent users from requesting facts, for instance, a hiring manager may wish to see the number of new hires or the number of applicants. If this fact isn't available it's going to have to be created or inferred based on some other field such as Hire Date.
It's also important you understand the process that underlies the OLTP database. With BA tools you can document this process. I had one situation where a billing process included dynamic revenue types. The user wanted to analyse and report on revenue. This required master data to be added to the ETL pipeline. Not a small feat. It's things like this you want to know about early on.
There's a lot more that I won't go into...currency translation, cost allocations, complex measures.
It's vitally important these things are well understood before development starts.