I think D&D 4e is deadlier and agree with the posts above that think so, I like to add the following to:
1) The combat involves much more movement and such a need for greater team tactics. Characters can do more than their 3.x counterparts, even at low levels, and monsters are easier for the DM to run. Mixed with the special abilities monsters have that Henry mentioned, this makes combat as a whole able to catch the players off gaurd. Then when they do not pull themselves together, characters get killed off quickly.
2) Healing Surges add another level of danger into the game and make characters more mortal. I've had players get in over their heads unexpectedly (i.e. like from situations described above) and then use up all of their healing surges keeping themselves alive (since Clerics and Warlord healing uses up the target's surges - which is so cool). Then the player's avoid the monsters, find a place to get a breather and regain encounter powers, but their healing is gone. Their at half hitpoints, dailies expended, out of healing potions and trapped in a bad spot where they have to fight their way through minions and the other monsters. Very intense stuff.
I also like to add, that the death saving throws are very dramatic. When someone goes down and is dying the panic and drama that takes over the players is very cool. They fight their way to their allies to stabize them (unlike before when they would metagame). This adds a level of danger too, because it makes the players more reckless and take chances when someone is about to die.
I have to add (with not flaming or starting any edition wars) that with these changes alone being discussed in this thread (4e combat being more dangerous) is one of two main reasons I could never go back to a pervious version of D&D. The combat is the most cinematic and intense I have ever seen it, without random save or die effects taking away the fun. Also, the game in no way feels like World of Warcraft to me (which I played for 2 years).
The other reason is how streamline and fun DMing 4e is, absolutely in my opinion the most fun I have had running games since I was a kid and I am 34 now and being playing since I was 10. I personally think WOTC did a great job. The only thing missing is the awesome campaign setting materials of 2nd edition (Dark Sun, Planescape, Spelljammer, Ravenloft) and some of the classic modules and content like Tomb of Horrors, Temple of Elemental Evil (etc). Give them time and I sure that stuff is coming (as we've seen in the Dragon and Dungeon articles).
With these two reasons alone, I look forward to running 4e every week and our group already has some great encounters and memories made using Keep on the Shadowfell (which my players should be finishing the last two encounters this next week). Ironically, I am the sort of DM who never used minis and steered away from "typical" dungeon crawls. I am very story and roleplaying focused and leave my miniature gaming with Warhammer Fantasy Battles (which I've played for 16 years). D&D 4e I think is the first RPG that successfully uses minis to improve the combat encounter more than it hurts the immersion. The group tactics, near death elements and hordes of monsters swarming the players that come from it are awesome.