I don't think D&D Next is espousing simplicity for simplicity's sake per se, but rather an overall design methodology to make sure the game never, or at least rarely, bogs down into rules quicksand.
Take a look at the four main mechanics of the game: the attack roll, the saving throw, the ability check and the contest.
I am hard-pressed to find a situation where one of those four rolls would not sufficiently resolve things in a satisfactory way to everyone at the table. If you can think of a circumstance where this over-simplifies the situation so much it become unrealistic, disruptive or cheats a player out of something deserved – please elaborate. I think those four types of rolls will cover 95% of the situations that might arise. And if it means some realism is sacrificed for the other 5%, then that's a price I'm willing to pay for a fluid, fast-paced and immersive game.
Bear in mind the past two editions have been horrendous as far as game speed and rules bloat goes, so you can expect the pendulum to swing back the other direction, which isn't a bad thing at all.
We'll see what it ends up looking like; in the end, swinging back to far may end up hurting them just as much. Simpler rules require a much more skilled DM that is used to maintaining internal consistency without external help, and the number of people who could pull it off in the pre third edition era was small enough; matching even that small number today would be difficult, and that means that a lot of people are more likely to have truly bad experiences that completely turn them off, whereas with 3.x and even 4E, there were at least some limits that would keep a mediocre game from turning into a horrible waste of time.
I never had much problem with game speed or rules bloat with 3.x/PF, as I found taking 5 minutes here and there in between sessions to make sure that I was good on the rules I needed for my character was ample, and a lot of the stuff that was inherently complex like crafting and leveling up could be shifted to emails in between games. I don't have it with PF because I continue to do the same thing as a player and as a DM I insist that my players do likewise. They don't have to read the entire book, but I am not there to run their character for them, and I won't; if they want to use a strange or unusual mechanic, it's on them to understand it forward, backwards, and sideways.
What little I played of 4E basically shifted it back all the way to the point I couldn't go five minutes without playing 20 questions with the DM, but it was definitely a simpler and faster paced game, so don't assume that fast paced and simple automatically leads to immersive or fluid. It's as immersive as the DM makes it, and there aren't very many DMs out there that can truly pull off a true rules lite system, or even a psuedo rules lite system like 4E. And unlike in a more rules heavy system, there's no safety net; one bad DM, or even a mediocre one, could easily turn a player away from being interested enough to try again, whereas with 3.x/PF, you have to work to have a truly horrible experience with the system itself, making it less likely to drive people away permanently after one bad session. Most of the stuff that people scream about being broken or overpowered is simply a case of someone didn't bother to read the entire description or applicable counter measures, not the system itself.
The biggest misconception I see with rules heavy systems is the idea that you have to learn all the rules and that you have to keep up with all the supplements, especially as a DM; the reality is you don't, not even as a DM. You learn the basic stuff, and if a player wants to do anything new or exotic, it's their responsibility to learn it properly so that they can teach everyone else as needed. If they want to be a grapple monkey, it's their job, not the DM's, and not any of the other players, to learn those rules by heart so that when they come up, the required rolls can be whipped out, outcomes determined quickly, and the scene continued. This does mean that everyone at the table has to do at least some reading, but if you aren't engaged enough in the game to at least know your own character and how their capabilities work, you probably need to make a simpler character or find something else to spend your time doing that you actually enjoy enough to put effort into. I've sat at enough tables to understand that the vast majority of the time that people just show up and roll dice when told to aren't bringing anything to the table and limit the experience for everyone else due to their inattention and apathy, so I have a hard time accepting reasons why people shouldn't have to read at least a little and put at least some effort in between sessions.
If Next can find the sweet spot that provides enough rules and common background to allow players to fully take the burden of their own character on their own shoulders while keeping the game itself simple and reasonably quick, I'll be impressed. I don't see it happening though; 4E didn't, despite their best efforts, and the sweet spot for Next is even smaller, because it's competing with 4E as well.