jimmyzimms wrote:
However the general feel around these parts seems to be that THawk with assault and supporting devs is a risky but powerful air mobile attack combo.
Can confirm that it's hilarious fun air assaulting with an assault detachment w/ chaplain and devastators w/ librarian. I enjoyed it anyway

Adding to the advice above...
Space marines have the speed to dictate where the fighting takes placeUse that speed to your advantage and pick your fights carefully and only assault when you're confident you can win and/or can prep the opposition for assault as mentioned above. Don't want to get into an assault with that big stompy death unit that's pretty much sat on your blitz? Then don't, run away and come back later on.
I learned this the hard way in my first game vs planetfalling world eaters, they dropped from a spaceship and landed near my blitz. My reaction? Assault! I threw a terminator detachment, tactical detachment and some scouts into the fray only to have them torn to shreds in turn 1. If I'd thought it through, I should have just moved them well away until I could get some support on the board, the planetfalling troops had no transport so wouldn't be able to give chase anyway so wouldn't be a threat.
Get the most out of your detachmentsThink of it this way, to use the tactical detachment from Redcar's example, on their own they cost 300 points. You activate and put out a bunch of shots with them, if they're not in a position to support an assault, deter opposition assaults, etc. then you've only got that single activation out of their points cost for that turn. IF you get them into a position where they can ALSO lend support to an assault then they've just become more valuable and you're getting more bang for your buck so to speak.
Don't lose sight of the objectivesAs Ginger said, the tourney scenario is won on objectives.. you could spend the entire game avoiding fights and grab a bunch of objectives in turn 3 to win. Not likely to happen but it highlights that you should always be thinking about the objectives, not how much damage you can put out.. yes I'm sure it'll be fun knocking out your opponents prized titan / gargant, but is it going to help in grabbing an objective?
Don't commit too early, play cageyThis goes back to the first point about dictating the fight. Don't be afraid to hold back terminators, thunderhawk ground assaults, etc. - it's likely better to keep your opponent on his toes knowing that you have a hefty assault savvy unit or two in reserve ready to plonk down than throw them into the fray early on and get them butchered without support. Plus throwing down a handful of terminator detachments on the start of turn 3 is always good to get your opponent sweating.
Think I've repeated a few points from above, but added my own thoughts to it too, hope that helps

ninja edit: I should mention that I'm yet to win a game with SM, but it's beginning to come together and I feel like my understanding of how they play is vastly improved from that first game I played with them.