The biggest problem D&DN has is really very simple. I think it has really missed the boat on several things including being modular which was apparently a design goal for the edition. Over the last 6 months or so I have been playing several d20 and D&D editions which was similar to 2010 or so when I was playing a mixture of 4th ed, Pathfinder, and Star Wars Saga. I have also been reading through my D&D collection paying attention to my 2nd ed , 3rd ed and 4th ed books along with adventures, from Gary Gygax's work through to the Kingmaker Adventure Path from Paizo.
And here is the main problem with D&DN. It is boring and bland. The nicest thing I can say about it is that I do not dislike it drastically. 2nd ed is very pleasant to read about and it has a modular via optional rules. 3rd ed has the basic d20 mechanics which was one of the best things to happen to D&D while 4th ed has balance, powers etc and at least does what it is designed to do very well.
D&DN is a simplified version of d20 which is fine. Bounded accuracy however s boring and seems to contribute to this via class design. Fighters for example get +5 to hit over 20 levels and they have dead levels. Even in second ed which was very simple you had a +1 to hit bonus to look forward to along with increased saving throws, weapon and non weapon proficiency's. Sometimes things may make sense mechanically but you have to sell this to someone and it has to be commercially viable. Even if you dial down the numbers porn of 3rd and 4th edition D&DN is struggling to compare well with 2nd ed. If 2nd ed had BAB instead of THACO, Fort/Ref/Will for saves and the 4th ed DMG and combat chapter I would probably buy that over D&DN.
This is where the numbers came in. 2nd ed had to many extremes at higher levels between the fighter THACO (BAB more or less) and every other class. 3rd ed did kind of fix this but still ahd some issues while 4th ed made every class have he same scaling modifier which did not sit well with fans of older editions. Star Wars Saga more or less used +20 over 20 levels and +15 for the combat weaker classes. +5 is probably about right IMHO although you could narrow that down further.
D&DN is supposed to be modular as well but so far it fails at that. Star Wars Saga used a very simple class progression for all of the classes that was talent, feat, talent, feat. I would not recycle this for D&DN but I would look at it for a few classes and drop the feat part.
Note you can extrapolate this all the way to level 1000+ if you want to although BAB would need to be capped. No dead levels, and a very simple progression. A talent can be more or less anything you want so if you add in bonus feats you have the 3.5 fighter. Add in powers you have something similar to the 4th ed fighter. Add in some one combat options and you have something a little like the 2nd ed fighter. Have a unified talent/maneuver section and it is very simple the downside is one can have 20 options by level I suppose but its not that bad compared to all the feats, spells, powers etc of 3rd and 4th ed and even the 2nd ed Fighter would have a similar number of weapon and non weapon proficiency's. It may not appeal to the BECM players, but the 2nd, 3rd and 4th ed players probably outnumber them as a safe bet. The design is inherently modular.
Classes like Rogues, Wizards etc have +15 to hit over 20 levels. This is where bounded accuracy comes in. 1st and 2nd ed kind of had this, the concept is not new. AC's topped out at -12 (32 in d20) for a great Wurm Gold Dragon. A Fighter naked would hit this on a roll of 12 or better, while the other classes would need a 17 or so. This is perfectly fine and the Dragon would not need a bucket load of hit point to survive for a few rounds. A +5 weapon which BA RAW in D&DN cannot seem to handle would make this fight easier so the Dragon would need some minions or a way to get the AC up to 35 or so to compensate (a buff spell with concentration). This lets a low magic setting have the Dragon as a major menace especially if he is resistant to non magical weapons.
One may have to tweak the encounter a bit if the PCs are loaded out with +3 or better weapons but that is also fine. Right now BA is so low that level 1 PCs can luck sack their way into hitting DC 25 skill checks, Rogues murder anything non combat related via skill mastery. Bounded accuracy as a concept is fine but it needs to be stretched probably into the 30-40 range. This keeps the numbers under control and allows class design to gain things like bonuses to hit at every level while avoiding the infinite numbers of 3rd ed and the hit point bloat of 4th ed. A narrower range of BAB/THACO between the classes whatever fixes the problem it had in 2nd ed.
Skills can also be stretched out as well even if it is only d20+ your ability score instead of your ability modifier. The skill system in D&DN is terrible right now due to multiple reasons. The skill dice and rogues being the main problem, with BA being another as the ranges are to narrow and it a leads to boring classes with little sense of progression or of getting better like the skill/proficiency systems in 2nd, 3rd and 4th ed all had. We have to buy this beige Toyota at the end of the day and I prefer a red Porsche. D&DN is not bad, but it is worse. It is boring. Maybe it is designed for new players, I have my doubts as D&D is a niche hobby. All other editions of D&D including BECMI and 1st ed had a greater sense of progression and that is the main problem of D&DN so far. And most of this I think is due to BA the one mechanic we have no real say as to if we want it or not. Right now that by itself is probably enough to kill D&DN for me although I have committed to buying D&DN core rules to reward WoTC for at least trying and putting up PDFs again. D&D is many things to many people. Bland should not be one of them.
Fear is the Mind Killer