Everybody changed big time with the new edition, and so did our favorite red-headed stepchild. Daemons aren't getting a lot of press these days, but I think they are one of armies that received the most buffs. I'm going to do a more comprehensive overview as I get more practice in, eventually creating a new guide for 6th edition, but here are the quick hits.
The biggest buffs to Daemons:
Daemons can now reliably tear up tanks in combat, hitting on a 3+ at the worst. This lessens our reliance on Bolt and just makes Fiends that much better. Hull Points make life easier as well. All this means that mech doesn't ruin our day all the time any more. That's reserved for something else.
No Retreat is gone, commence the dancing in the street. We no longer lose wounds on MCs from lesser Daemons getting slapped around. This is a huge boon to Slaanesh Daemons and Nurglings, who are cheap, fragile and numerous. Now once Daemons are in combat, they'll stick it out to the end, for better or worse.
Reserves and Deep Strike
Reserves comes in on a 3+ every turn and automatically on turn 4. That is huge in mitigating the negative effect of Daemonic Assault. Now expect to have 2/3 of your second wave to arrive T2, essentially giving you 5/6 of your army and almost all of it by turn 3. Furthermore, Deep Strike is much safer and more reliable.
Daemon Troops aren't so hot, so maybe add in some Guard, CSM or Tau to buff you up. Flyers causing a problem? Field fortifications or allied units. Allies give you a whole new level of flexibility in your list.
Of course, GW giveth and taketh away.
This is the bugaboo of 6th edition so far. Flyers are very much an all-or-nothing deal, dealing lots of damage while being very durable unless you have dedicated anti-air. Daemons don't have that, obviously. Our only anti-tank is the single shot Bolt. The best way Daemons can handle aircraft will be Vector strikes from Greater Daemons (particularly Bloodthirsters) and Daemon Princes, fortifications (lolwut?) and allies, particularly Guard and Tau.
AP3 blows. Bloodletters, Bloodcrushers and Khorne Heralds are boned. Boned.
Now most cover is no better than your typical invulnerable save. Unless you're hiding in ruins, Daemons are much more vulnerable to getting blown off the board by shooting. Bigger units or closing quicker are very important.
Charging and Overwatch
Random charging without Fleet is not terribly reliable beyond 6-7". Overwatch can potentially kill a charge, particularly a long one. You'll have to get closer for reliable charges and will need more than 1-2 models in range.
Overall, more good than bad I think. The positive changes have a much larger impact, really saving Daemons from the scrappy heap. Time will tell obviously, but I think things are looking up.