What's the hardest or most misunderstood aspect of LINQ? [closed]

static void Linq_Deferred_Execution_Demo()
{
    List<String> items = new List<string> { "Bob", "Alice", "Trent" };

    var results = from s in items select s;

    Console.WriteLine("Before add:");
    foreach (var result in results)
    {
        Console.WriteLine(result);
    }

    items.Add("Mallory");

    //
    //  Enumerating the results again will return the new item, even
    //  though we did not re-assign the Linq expression to it!
    //

    Console.WriteLine("nAfter add:");
    foreach (var result in results)
    {
        Console.WriteLine(result);
    }
}

Before add:
Bob
Alice
Trent

After add:
Bob
Alice
Trent
Mallory

Single(x => x.id == id)

Where(x => x.id == id).Single()

[TestMethod]
public void QueryComplexityTest()
{
    var users = _dataContext.Users;

    Func<User, bool>                funcSelector =       q => q.UserName.StartsWith("Test");
    Expression<Func<User, bool>>    expressionSelector = q => q.UserName.StartsWith("Test");

    // Returns IEnumerable, and do filtering of data on client-side
    IQueryable<User> func = users.Where(funcSelector).AsQueryable();
    // Returns IQuerible and do filtering of data on server side
    // SELECT ... FROM [dbo].[User] AS [t0] WHERE [t0].[user_name] LIKE @p0
    IQueryable<User> exp = users.Where(expressionSelector);
}

from outerloopitem in outerloopitems
from innerloopitem in outerloopitem.childitems
select outerloopitem, innerloopitem