Aaron Bertrand (@AaronBertrand) shows a quick demo that illustrates why you should be very careful about oversizing varchar / nvarchar columns.
Paul White (@SQL_Kiwi) takes you on an optimizer journey, exploring how SQL Server comes up with cardinality estimates for COUNT queries.
In an effort to make leading wildcard searches sargable, Aaron Bertrand plays around with a trigram-type implementation in SQL Server.
Paul White shows that SQL Server sometimes produces very misleading information about the impact of sort operations.
Rob Farley (@rob_farley) digs deeper into seeks and actual rows read, demonstrating a case where you want a non-sargable expression in the WHERE clause.