What traits / skills does the Business Intelligence Developer role expect over a traditional Web Developer role?
First of all, let's define Business Intelligence. It's simply trying to make sense of the data a business already has.
As an example, consider a company which sells toys, and stores a record for each toy it sells. This record contains the information of the country in which the toy is sold.
Now, the manager of the company wants to see in which country the sales is higher, so that the next year, the distribution plan would be more efficient for that country. He/She needs a report of the sales figures in different countries. This is an example of business intelligence.
Now to get to this report, somebody has to get the data out of the database (storage place, anywhere, even an Excel file). But wait, what if the total records of the data you have in your company exceeds, say for example, 50 million records? Do you want to query over them each time you want to create that report? Even worst than that, what if your database is under a huge amount of transaction and many records are getting inserted into it, while you try to execute a very costly query upon it?
These problems resulted in some science to grew out of the solutions developers proposed. For example, you might create another database, and run a job each night to replicate these databases, so that tomorrow you can execute your query on a database which is not under live transactions.
Some concepts come to mind here, like OLAP (Online Analytical Processing) vs. OLTP (Online Transactional Processing), Data Warehousing, Data Mining, Cubes, Tools for BI like SQL Server Reporting Services and SQL Server Analysis Services, and many other concepts, which are not related to being a web developer at all.
A BI developer may actually not have any web development know-how at all.
Core skills of a BI developer would be in-depth knowledge of databases, data mining, data warehouses, etc. There's probably a bunch of proprietary systems (e.g. Oracle BI Suite) that a BI developer would be expected to know (and have specialized in one or two of them).
Domain knowledge of the business in question would be a big plus as well.
A microsoft BI-developer would know most of these:
SQl server 2012 is the latest and the greatest database from Microsoft.