That's about it. Rather than a long rambling rant, or some stream-of-consciousness gestalt about some list or codex or splatbook, I'm going to try to keep it simple here.
Overthinking is just as bad as - and possibly worse than - underthinking.
First, let's start with army selection. When you first enter the hobby, you may find yourself dazzled by the array of armies before you. So many models, color schemes and special rules can be overwhelming. Well, relax. You're first impressions are probably correct. You think Space Marines are awesome? They probably are. Do you dig the nids? Then that's the army for you.
In fact, the worst thing you can do when focusing on your initial army selection is focus too heavily on the rules. Going back to my original Hype article, too much information can be a bad thing, particularly for a newbie to the 40K scene. Listening to the griping of other, more experienced players about the shortcomings of this army, or the glaring inconsistencies of that unit can only limit your enjoyment of your first army. The shine will be off the apple before your first bite!
This logic applies right down the line.To painting, to list-building, and right down to standing at the table throwing dice.
Sure there are "first-timer" armies like vanilla Space Marines, and indeed Space Marines make an excellent introduction to the game, but at the core of this immense hobby of ours is a sense of identity; finding the army, the list, that fits you personally, your style of play and sense of visual style. Without that, the game falls flat.
When it comes to strategy, it's also very easy to overthink. If you try to analyze every outcome of every decision, you may find yourself with nothing but corners to be painted into! Particularly when you are first starting out, you just have to throw yourself in there, think about basic tactics and synergy, units supporting one another, and the rest will begin to flow naturally as you evolve and learn more about the game.
So stop thinking so much. Play.