On scalability and memory footprint
wheels: "We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil." -Donald KnuthThat said, watch your 'n's.  If you scale linearly, you can keep throwing servers at the problem.  If you end up with some critical piece of your code that's, say, quadratic in run-time or memory consumption, you can't.  And when you get there:  profile, don't guess.  Monte-Carlo code optimization is the beginning of the end for an otherwise clean code-base.