For me, the killer of the current "iteration" of the playtest is the skill system, which I am hating with all of my hate. All the RPGs I like to play are highly skills-focused: Dungeons & Dragons 3.5, Mongoose Runequest (2/6/Legend), Dark Heresy, Rogue Trader, Eclipse Phase, and WFRP 2e. One thing in common among all those RPGs is that they have a detailed and granular skills system where each skill has listed skill uses with a set target number and game effect, and a character is defined largely by their skill set.
Many people dislike such skill systems, citing that they don't need rules for roleplaying and that the skill rules get in the way of their game and to this group Next caters, but for me such a skill system is a vital component. Skill uses act like "powers", giving the player a game piece to manipulate in order to overcome a challenge. You don't need rules for roleplay, but you do need rules for a game. In Next, as it currently is, skills are just a die roll to beat a number that the DM must decide arbitrarily and the effect of the skill's success/failure is also in the hands of the DM. Without some predictable uses of a skill there is no tactics involved in deciding to use it, or deciding to train in it. For me, if only combat has rules for it then most of the game is still missing.
Another issue I have with Next is that there are less things each character can do. I prefer rules-heavy systems because they give more pieces to move, so to speak. In short:
They are never going to please everyone with 5e.
The real question will be: can they please enough people to turn up a profit?