And using IEnumerable was probably making the point that it didn't have to be a list, it could be anything implementing that interface.
Downvoters be damned, they know not what they talk about. ultimape is clearly not a C# developer.
There is a lot of discussion about the usage of var, and ultimape's non-usage of it in his example is a personal preference, as well as making sense in an educational post.
And lastly, I think you'll find many developers, young and old alike, use x, y and z as their lambda parameters. I do too. It comes from the mathematical function origin of the syntax. In most early examples when it was introduced, and loads of educational materials now, lambda syntax is still presented akin to a mathematical formula:
x => x+1
In that context, using x makes a lot more sense than p, and has stuck around as a habit. It's a style thing, and everybody has their own style. You don't get to claim superior experience or skill based on a 2 line snippet that doesn't match your style.
And as an aside: you're not being downvoted by "the ignorant masses". You're being downvoted for being confrontational and not contributing to a conversation about the topic, which is Parallel LINQ and not coding style.
For longer queries, that little DSL syntax it's a bless. It's way more pleasant to read, write or think about it than the alternative.
Ah, and since you care so much about it, I've been developing in C# for freaking years that I don't bother to sum it up anymore.