This is by no means an exhaustive list, just some things (good & bad) that stood out from my first pass through the new SM codex. In no particular order:
I don't think Calgar is worth his points (again); those traits aren't that good, even using the best one twice.
Khan is crazy good for the points. He basically got better and way cheaper. And trust me, I'm not complaining; I thought the old Khan was great... I just find it hard to justify the points on some of the other named characters when you compare them to Khan and what he does for your army.
For example, Shrike. I still don't know why they price him so high (this was the same issue I had with him in the last book). He's only 10 pts cheaper, but now he can only join other jump marines at the start of the game... which is pretty lame. I also don't see what all the fuss is about regarding Raven Guard chapter tactics. Scout/Stealth doesn't affect bikes, termies, centurions, or even the iconic RG squads (jump marines!). I do like being able to use the jump pack to move and assault, and re-roll hammer of wrath is nice. But with all the ignores cover in the game now, who really cares about stealth (first turn only) that only applies to your infantry squads. I don't know, I guess I'm let down again by RG rules (really wanted jump marines as troops and/or scout bikes as troops since we're also a big scout army... or at least wanted both of those to benefit from Scout; again, better off using White Scars chapter tactics).
Moving on, if I did want to run a RG themed list, at least I can take Land Speeder Storms as dedicated transports. I also like how they moved Vanguard squads to the Elite section; that's 2 units that previously ate up Fast Attack slots. Both are a little cheaper too. Pretty happy about these changes.
Bikes are going to be amazing. Cheaper and one of the very few squads that have access to multiple Grav guns (and I mean very few squads... Command Squad can only take combi version, which is ok, but kinda shocked they took away all normal special weapons options for this squad... guess I'll need to convert those plasma dudes into combi-gravs, haha). But yeah, White Scars bikes are the new hotness.
Since I just listed a positive, here's another negative: Stormtalon can no longer Escort an Outflanking unit. Pretty disappointing since I planned to do just that. Balls
Legion of the Damned are now playable, I think... At least in some lists.
The new tanks are nice, both the Stalker and the Hunter. I'm going to give them both a try at some point.
The devastator centurions armed with Grav-cannons & amps will be pretty nasty. Expensive, but nasty. Just imagine all the dirty tricks you can do with them when you roll the right psychic powers (which Tigurius all but guarantees) or you add the right ICs.
And it's already been said, but the Thunderfire is going to be making a lot more appearances. Such a good unit for only 100pts.
Overall, the book is solid. Definitely not over the top though. It feels a lot like the 5th edition book, but in my opinion, that was a fantastic codex that held up pretty well throughout the life of 5th edition, so I didn't think they needed to overhaul it, just update the points to 6th edition. I think there is a lot more to being able to ally with yourself with a different chapter tactic, and I'm sure there are some very deadly combos to be found with that alone. But that's all I have for now, time to sleep on it!