One of the most common trends that I see related to performance & scalability with SQL Server is a poorly configured or implemented storage subsystem. There is a ton of information out there on this subject but in my opinion that is part of the problem. Too much data is not always a good thing and there is a lot of misinformation out there as well. I also see a lot of systems that were configured based on the advice someone gave them or they read about which may have been great for that other system but not necessarily for theirs.
So I think it is time we got back to the Basics and Best Practices when it comes to I/O in SQL Server. Again in my opinion and my experience from seeing systems all over the world this list should get people off on the right foot if they are not sure what they need or how they should approach a proper I/O configuration. The first article hits the nail right on the head and is a great place to start. The next two give a very good understanding of what actually goes on when SQL Server makes I/O requests and explains the terminology so that everyone can talk the same language. The 4th link is a relatively new white paper to most that should be sort of a bible and gone over long before you deploy or even buy the equipment for your next SQL Server. And finally there is a link that everyone should be aware of that gives you access to a whole host of white papers that should be read as needed.
SQL Server Storage TOP 10 Best Practices
SQL Server 2000 I/O Basics
I/O part 2 for updates to SQl2000 SP4 and SQL2005
SQL Server I/O Pre-Deployment Best Practices
Overall SQL Server Best Practices