Overall I like 6th edition 40K. There are some wrinkles, but for fun games with fun opponents I think you can have a great time with this system. With fun lists and terrain and missions set up to tell a particular story or a campaign I think it will be fantastic. Random pick up games or tournament games with random people using random objectives and random terrain etc etc, can still have fun but it's not using the whole potential. For those with a dedicated gaming group that are into narrative gaming this will be a golden age of 40K. With promises of more terrain, warlord traits, missions and the like to come in White Dwarf we may also see a return to some of the fun and free-wheeling of the old Chapter Approved days.
As for specific rule changes, I'm shocked at how much the pendulum has swung towards assault armies. Units arriving from reserves not being able to shoot is crazy, and makes a number of units who rely on shooting pointless to outflank, as they'll just get assaulted off the board after they arrive. Not being able to shoot after disembarking even from a stationary vehicle is huge! Being able to assault nearby units that shoot at you in the opponent's turn isn't too bad on its own since your swings only hit on sixes, but it still influences the balance. And not being able to shoot flyers at all due to the speed but being able to assault them is just another insult to shooty armies. And that's all not even factoring in the random firing range for shooting attacks or half the plasma weapons moving to AP3!
Of course, the above is a joke - for the reality, flip shooting and assault. Assault still has its uses - it keeps the enemy from using all those guns to shoot you even more, it can get you closer to objectives, etc. - and there have been some changes to improve assault (killing vehicles through assault is a lot easier, decent chance for night fight in all standard missions) but on balance I think things have moved even further towards shooting. I think that assault armies may still be able to do well, but you're going to have to really plan ahead when playing them, from terrain placement on up.
Call me a hypocrite after saying that, but I do like some of the changes to shooting. Full-range move and fire rapid fire, snap fire moving heavy weapons, keeping heavy guy stationary while rest of squad moves, snap fire while gone to ground are all nice, and while in many cases the impact on the game is small, the impact on the entertainment is higher, as your units are more involved and have a chance, even if it's small, to stay involved in the battle in more cases. On the same note, more forgiving regrouping is also good, though I do fondly remember shepherding lots of enemy units off the table back in previous editions.
I've enjoyed the 6th ed games that I've played so far, and look forward to more, especially ones that I can arrange in advance. I look forward to playing my various armies under the new set, and though there may be some tweaks warranted to some, I don't think any have been crippled by the changes and some even may have boosts. I am also certainly looking forward to the new Chaos Space Marines codex and will enjoy building and playing all sorts of stuff from there, and playing against it with my loyalist armies. I just need to finish up these Space Sharks to clear my painting que for Chaos!
On another note, I think the new rules provide a great opportunity for 40K podcasts. While some podcast titles are pretty straightforward, like 40K UK, some pull from the miniatures (The Heroic 28s) or loosely from the background (The 11th Company or The Imperial Vox Cast), others are not directly 40K related, like the Overlords. There are also a lot that pull their titles from the 40K rules, like Deepstrike Radio, The Independent Characters, The Preferred Enemies or The Eternal Warriors.
It's here that I think 6th ed introduces some good new fodder for prospective podcasters. If you're starting up a new 40K show, there may be some good inspiration. Maybe you can invite listeners to jump into the Wound Pool podcast. If your focus is to help tournament players be aware of rules disputes, power armies they may face and registration deadlines perhaps Look Out Sir would be a good title. The Insane Heroes sounds likely (maybe Adamantium Will can be one of the hosts), The Hammer of Wrath podcast can bring you news fast (at I10!), The Soul Blaze podcast can cover controversial topics that keep on burning, or you could lead the Warp Charge while making good Hull Points. Of course, if listeners (or you!) can never be sure what you may be covering from episode to episode, perhaps Mysterious Objectives Radio is for you. Perhaps you'd even like to Deny the Witch or make a Psychic Shriek across the internet. I look forward to listening!