[profquail]

Counter-argument: maybe by completely banning LINQ you're prematurely optimizing? If you can describe some query or aggregation with LINQ, it may not always be super-fast, but it's also less error-prone than hand-writing equivalent, faster code. It seems like it'd be better to allow the usage of LINQ, at least for the purposes of rapid prototyping and implementing behavioral/unit tests; once you're happy the system works as expected, you can hit it with a performance/memory profiler to identify specific parts of the code which need to be optimized.

Also, check out LinqOptimizer -- it analyzes your LINQ queries and can optimize them for better performance (and it's free!): https://github.com/nessos/LinqOptimizer