My goal as a GM has always to make the game balanced and fun for all players involved, and in that regard 5th has almost done well SO FAR. And it looks like you removed MOST of the balance issues and the inpedements to a fun play-session. Unfortunately some of these things look like they could potentially fall back into that old not REALY fun but just fun enough to not try anything else at levels beyond 5.
1. Save or Die/Charmed/Paralized or Otherwise Helpless [to be refered to as save-or-(done)]
-- This, to me was the death of every 3.x campaign I ever ran. When the power to end an encounter with a single roll became accesible to the players, it made everyone without that ability completely impotent. If you were doing damage that had an actual number attached to it, and not making something (or multiple things) take saves to avoid being flat-out out of the encounter, then your actions became meaningless. When the power became available to monsters, the same was true (and it seemed like the developers knew that, because you would be hard pressed to find a 3.x monster of CR 11+ that attacked in any way other than forcing players to make a save or be out of the fight.) Partially because saves auto-fail on a 1, and partially because ANY single d20 roll making a fully fleshed out character go from awake and at full health to making that player re-roll next session. I saw player after player become disheartened and stop playing at that the exact point where save-or-(done) powers became a reality time and time again, it is probably the second most common reason for a game not making it past 11th level (the leading being ADD.)
When you re-introduced saving throws in the playtest, I got worried. Fortunately, I did notice that there were absolutely NO powers resembling save-or-(done) mechanics. The closest was the sleep spell, which is just mathematical a high damage all-or-nothing finishing move. This is acceptable, because it means if someone is taken out of the fight by this, they are either SIGNIFIGANTLY below the level of the caster of the finishing move. The problem is that other than sleep and color spray (and some splat book powers not worth mentioning) most save-or-(done) abilities/spells/powers/etc... weren't introduced until level 7+ (the same time most people stopped playing games that start at lv1... think about it.) PLEASE PLEASE PLEASE, if you insist on keeping saves as a mechanic (and I'll admit most of the players you lost with 4th wouldn't be happy without them,) keeping them all as high damage finishing moves instead of single die save-or-(done) abilities.
Bear in mind that it is imperative that ALL and not just SOME potential save-or-(done) abilities, because if one class has access to them, and another doesn't this just increases the imbalance issue.
2 Full-Attack actions
--This one takes less explaining. Yet another frustration to MANY players through the course o 3.x was the fact that, beyond a certain point, unless you were a caster, if you wanted to do big boy damage in a turn, you had to be standing still. The absence of this (in my opinion) the biggest positive difference between 4th and 3.x: tactical combat. Because damage still mattered [thanks to the absence of save-or-(done)] and because said damage was not significantly reduced by not moving, you had a MOBILE combat experience. In the current version of 5th/next/the playtest, also has a mobile combat experience thanks to the absence of Full-Attack actions, save-or-(done) and one more feature (to be discussed below) . The only problem is that for most people, full-attack-actions didn't start mattering until after 6th level (the notable exception being two-weapon-fighting, which works fundamentally different now. Please don't include full-attacks after 5th level
3 The ability to negate attacks of opportunity
--As I mentioned above, tactically mobile battlefields are fun, and they make the combat experience fun in and of themselves. One of the things that made tactical mobility matter in 4th as opposed to 3.5 (which was fun, but the combat wasn't fun in and of itself.) However, full mobility can be just as boring and static as no mobility, and the ability (everyone always being able to attack everyone is just as boring as only being able to attack the person you charged or who charged you first turn.) There seems to be no abbility to move without preventing attacks of opportunity without doing something equivalent to a 5 foot step or a shift. This is good. This is important. Of course, without the above two qualities attacks of opportunity cease to be relevant because they don't do full-attack-level damage, or make the opponent make a save to avoid dying.
4 Immunity to types of damage, particularly percision damage
--Save-or-(done) is the worst kind of "you don't get to play" anymore situation, but there is another kind of "you don't get to play" situation. Have you built a fantastic rogue/scout only to find out that the campaign is going to be one in-which you primarily fight undead, constructs, plants, and oozes? Of course you have, and I've watched that player quit... or at least I would have if I hadn't have the house-rule that nothing is ever immune to percision damage, ever. In a world where damage actually matters, completely negating someone's ability to do damage is the same as saying, "you rolled a one on your first die roll today, and it was a save, so you don't get to play." The bigest difference, is that while you don't have to roll an entirely new character, you didn't even get a single die roll to save you. What's more, if you just flavor sneak attack a bit differently, then it makes perfect sense working against undead (aim for the brain against zombies,) constructs (clogging up the gears with your arrow,) and even oozes (leaking the bulk on the floor with a well timed and placed puncture.)
TLDR: Don't throw the baby out with the bathwater.
Edit: Edited for potentially flame war inducing edition banter