Know What Your Code is Doing to SQL Server!
ORMs - oh how I LOVE them! As a consultant when I see them in play at a client I go KACHIIINNNNGGGG!! :-D Seriously though, they can provide some substantial benefits for coding throughput. But if you don't Read The Fine Manual and do some things right and avoid some major gotchas you will fall prey to one of my favorite Guruisms: "Anything that allows developers to slap code together more quickly is inversely proportional to the performance and scalability you will get from that code"! Although this talk is based on Entity Framework, most ORMs suffer from a lot of the same flaws.