My bad, I expressed myself poorly; I was supposed to say "in my opinion" in that first sentence, but could have chosen my words better. Also, you're absolutely correct about the final rules; we haven't seen them yet.
You might feel differently, but IMO flexibility in 5E seems to come at a cost:
1) the 'basic' version of the core rules looks simple and streamlined, but lacks both depth and options. It might be a good thing for some, but for experienced players might be too bland (again, depends on what you prefer). And seems to require a lot of on-the-fly decisions, adjudication and improvisation on the DM's part.
2) the 'advanced' version of the core rules seems be that you plug a few modules into the 'basic' version to achieve more options and complexity (e.g. skills, feats and tactical grid-based combat). Probably requires group effort to vote on which optional rules the players want to use, which might naturally mean that each group uses a slightly different and/or house-ruled version of Next. I personally hated this back in AD&D, but I guess some people love to tinker with the system.
3) they opted to go with "natural language" and got rid of 4E's elegant power format and keywords. I've seen some really heated arguments (and some of them lasted for hours!) when players have debated with DMs about grammar and poorly-written spell descriptions. IIRC the most serious fight was about 18th level druid using Shapechange (3.0) to polymorph into a dragon he had only scried, but never seen in action. Another argument took place in PF, when there was a serious discussion about what "attack action" and "attack" mean in relation to certain feats. And I can't deny that it's far easier for everyone to understand how a feat works if it says "Add +[W] to damage whenever you use a spell with the keywords Fire and Weapon", instead of "You may add an additional weapon damage die to damage rolls when you're wielding a burning or flaming weapon." (which reminds me that there have been long discussions online about what constitutes as "wielding"

)
4) and also there seem to be quite, eh, loose guidelines for DMs on creating their own magical items. I guess they wanted to break the mold so that people could "go wild" and "make magic wondrous again". I feel a bit leery about this; there were lots of hits and misses with magical items back in AD&D; some worked, some didn't. Occasionally it was liberating, but most often I felt frustrated by the lack of better instructions. And there were times when an item unexpectedly turned out to be simply too good to let the PCs keep it.
Of course, it's way too early to tell how the finalized rules will turn out to be. They might be surprisingly different in parts, although I doubt it. If I were a betting man, I'd put some money on the last playtest version being very similar to the printed book. Whatever flaws or bugs they have recently found will likely be addressed via dials, switches and modules (e.g. "If you find saving throws in the game to be too brutal, add +2 to everyone's saves"). Naturally, I could be wrong, but that's what I think.
Wow, that is actually a really good question! Honestly, I don't know; to me it looks like Next is a step backwards is design philosophy, with a lot of influences from AD&D and 3E. I hate the whole idea of bounded accuracy, because I don't think the math is as solid as it appears to be -- certainly not as strong as it is in 4E. That is also a subjective view, but IMO the whole "Ghoul Saving Throw Fiasco" was quite revealing about how much effort they've put into it.
I prefer PP/ED-style (and hybrid classes) to 3E multiclassing and prestige classes. And I don't think these "super feats" are the best way to deal with "traps" or add depth to bland class features. Plus if you ask me, IMO the racial abilities/features have been a real mess.
There have been some aspects I liked in the playtest rules; (dis)advantage is a nice mechanic, and I really hoped they had kept skill dice in the game (too bad they apparently didn't).
It may be that it's just too different from 4E and Pathfinder for me to like Next. I wouldn't go as far as saying that I won't ever play it, but if it's anything like the last playtest version, most likely I won't buy the books. However, I love FR, so I'll likely purchase at least some of the regional books, maybe even adventures.