I really like the new Space Marine codex. Not because of price drops, or how awesome White Scars tactics are, but because of the inherent play variety it offers.
For me that kind of variety is what makes some books stand head and shoulders above others. For some books, when you see armies made from them, a lot of similarity in build, and therefore play style, will be evident. When you face them again and again it can get repetitive.
Other books though offer significant variety, or at least a decent amount. Take the current Imperial Guard for instance - you can run a foot horde that uses bodies and orders to get things done, a meched up force, an army that relies on a mass of artillery, air cav, or various combinations. If you're not pushing for some kind of optimization, you can see a staggering variety in IG armies. I like that, and could be happy playing 40K if the only army available were IG. The variety of possibilities (and the variety of models) can keep Blue on Blue matches still very interesting and varied.
So, Space Marines. They have taken this idea and advanced it to the next level. A few years ago Wargamescon did a mirror match event where everyone used the 5th ed Space Marine codex. More than that, everyone had the exact same list. Now, they did this very creatively to make things the same but still allow for a little customization, by using Sicarius (who has options he can bestow) and a Librarian (where the players had choice of powers) as the HQs. Everyone had the exact same army list, but with a little player choice they could all play somewhat differently, and would have different advantages and disadvantages against each other.
Now, you can do that with pretty much any Space Marine list that doesn't use a few particular units or any special characters, via Chapter Tactics. You could face a half-dozen different Space Marine armies, built identically as far as units, weapon loadouts, and wargear are concerned, and still have them play significantly differently simply by the choice of Chapter Tactics. I love that. Of course the variety gets even better when you vary builds as well, but the fact that you can have a number of significantly different armies with the same models, using the same codex...is fantastic.
I don't know if they'll be able to fully do the same level of variety with future releases, but I hope that they get close. I'd love to see identical builds of Catachans, Steel Legion, Cadians, Mordians, Tallarn etc. play differently. I'd love to see identical builds of Goffs, Blood Axes, Bad Moons, etc. play differently. Here's hoping!