What are the 'Gotchas' associated with tacking on a ToList() to a Linq-to-Sql query?

Question

This question led me to wonder the following:

What are the 'Gotchas' associated with tacking on a ToList() to a Linq-to-Sql query?

I'm equally interested in best practices and anecdotes.

Answer 1

Disregarding transactions:

using (PlayerDataContext context = new PlayerDataContext())
{
    var players = context.Players.ToList();
    MethodThatUnexpectedlyTakes25SecondsToComplete();
    Console.WriteLine(players.Count()); // Outputs a stale count
}

Answer 2

The big one is the loss of deferred execution. A linq query normally doesn't actually do anything until you iterator over the results. Calling .ToList() iterates over the results, and so will run the query right away.

Answer 3

Compromising linq-to-sql's ability to optimize:

using (PlayerDataContext context = new PlayerDataContext())
{
    // suboptimal
    var players = context.Players.ToList();
    Console.WriteLine(players.Count());

    // optimal
    var playerCount = context.Players.Count();
    Console.WriteLine(playerCount);
}