Okay, this is interesting in that I partially agree with the designers on both counts. However, I do feel some things need to be taken into account as we look at this new paradigm or we will face the law of unintended consequences.
On item 1, the whole notion of "no dead levels." This is very important for anyone with gamist tendencies. We all like "new tools to play with." I concur that new spells are "new tools," in one sense, but only if they allow the caster to do something new. Doing more damage isn't "new" - it's just "more," like getting a higher BAB, better Saving throws, or more hit points. Nice, sure, but not exciting.
I'm going to take a moment to rant on spell slots and preparing spells for a minute. The problem with it is that you have access to all these new spells, and it's great that your best fire attack now does 5d6 rather than 2d4, but what happened to the mage being able to do something totally NEW? The nice thing in a spellcasting system like True Sorcery (for example), is that as you level up, you learn new talents, and can not only cast better spells, but you have more types of spells you can cast. The improvements are very satisfying.
I remember in one of my 3e games, one of the other players and I were disappointed when we levelled up because we hadn't even gotten time to try out all of our new feats yet. So in that sense, we were levelling too quickly. We'd get several toys to play with, and before we'd tried them all out, we had new ones. This would be less of a problem if those powers scaled better. We had a lot of fun in our Midnight game, because using the powers of our heroic paths was "cool." They gave us something new every level. Unfortunately, that game died out as real life got in the way. We were clinging on, and barely surviving, but man, was it fun!
That issue of new toys is why we had issues with Castles & Crusades. Basically, after level 1, there's nothing to look forward to. Every fight is just "whack him." BOrrr-ing! I have no problem with C&C for a one-shot, but for a campaign, there's not enough there to really sink one's teeth into.
As anyone paying attention knows, my favorite game right now is Iron Heroes. Between stunts, tokens, challenges, and mastery feats, those classes get plenty of new toys to play with. An Iron Heroes combat feels exciting, even epic, even at low levels. Now, I haven't had a lot of play time with it, but I think everyone who has would confirm that. It is also balanced on a per encounter basis, which brings me to point 2.
Per encounter balance has good sides. Basically, you want a character to have the ability to pull out their "most powerful ability" only once every so often. If it takes time to make it work, they won't necessarily go to it just to wipe out a minor threat. Those, you just take out conventionally. The downside of saving up for your big ability works like this...you spend 3 rounds doing nothing, then strike decisively in round 4. If by contrast, you could have struck every round and been done with the same target in 4 rounds, you've literally gained nothing. When it is a benefit is facing the powerful opponent, where you can strike ineffectively for 9 rounds, just waiting to "get lucky" on the 10th, versus making delaying strikes, building up for a big finish. Your final blow is not so much the result of luck as of careful tactics. That rewards smart play and takes the whim of luck out of the scenario. Against some adversaries, if the characters never got lucky, they'd get squashed.
If you think about it, per day abilities (excepting hit points, more on that in a minute) don't make a whole lot of sense. Forget spells for a moment. Why can the barbarian only rage X times/day? That only makes marginal sense. Some of the other abilities, that are neither magical nor physically draining in the slightest, don't make any sense at all to be
limited to "X uses per day" - the Knights challenge ability (PHB II) for instance. However, the real problem isn't so much with those classes, which have other things they can do and will keep going. The barbarian doesn't make everyone stop when he's out of rages for the day.
The problem is the spellcasters. In my experience, they tend to burn through their abilities to make encounters easy, then make everyone rest for a day. Since nobody wants to go on if the cleric (especially) doesn't have any curing magic left, the game grinds to a halt when those classes run out of spells. The oft-cited "sweet spot" of D&D seems to be the levels at which the classes have enough staying power to keep going for a reasonable amount of time. Sure, most of the cleric's 1st level spells aren't much help at 4th level, but those spontaneously cast "cure light wounds" will help keep the party going through at least 4 encounters.
The game should be structured to keep play in the sweet spot. I'm convinced that the primary reason people like high-level play is that high-level spells and abilities are "cool." They usually represent new things you can do. There are monsters with more interesting abilities.
If combat could be made more interesting, people wouldn't feel the need for silly powers. How many players wouldn't get a charge being able to have the same effect on a battlefield as Gimli at Helm's Deep (taking out 50 of the enemy), Gandalf in Moria (fighting off the uber-baddy), or Legolas at Pelennor Fields (taking down an entire enemy regiment single-handedly), just to steal 3 examples from the Lord of the Rings films.
Since D&D parties adventure as a team, every one should have the same limit. As has been mentioned, people who want to keep the number of encounters limited have a very easy resource - hit points. If, for example, you instituted a simple rule that limited the number of hit points a character could recover in a single day, out the window goes the worry of characters facing every encounter "fully charged," so to speak.
Similary, you could have other rules that prevented spellcasters from "blowing their wad" of spells in every encounter. Ideally, to me, a spellcaster would resort mostly to reduced power spells, the magical equivalent of hitting someone with a hammer or shooting them with a crossbow. But they would be able to pull out their high-power abilities occasionally. If doing so drew on their hit points in some way (say by causing subdual damage), they would be limited by the same resource as other characters. However, if they burned through their high power abilities, they'd still have their low level ones. So there's no arbitrary stops. Characters would stop adventuring when they were too wounded to continue. Which might be significantly longer than an average person.
I'd recommend Reserve Points as presented in Iron Heroes or Unearthed Arcana as a wonderful way of extending character staying power and taking care of the "walking bandaid" problem, while not affecting per encounter balance. Basically, every character in Iron Heroes is walking around with an appropriate level "heal" spell that replenishes him to full capacity. It's the equivalent of the whole party walking around with 4 cure spells (of sufficient power to heal one character completely) in reserve. That's 4 CLW at Level 1, and so on. It is NOT the equivalent of a massive wand of cure light wounds that lets you keep adventuring until the spellcasters "run out of spell slots."
So I personally favor per encounter balance, with hit points being the only real "per adventure" limiter. I am in favor of any shift of the game to increase the fun factor and eliminate absurd situations like characters camping out in dungeons at 11 am.
My two cents.