I am a few days late from the opening post, but this is a topic that is of great interest for me. I have stated some of my opinions in another (recommendation) topic I have opened quite some time ago (and that is not being updated since most of my notes are on a laptop that needs repair):
viewtopic.php?t=8931
As in that topic, many of my opinions are similar to other users (especially bigfootkit), but I'll list them regardless of that.
So, let's start:
1. Extreme obedience to self-imposed rules, leading to lack of variety in sound
OK, this one has been discussed to death and is probably the single most cited reason why NWOTHM "sucks". I will simply quote what I have wrote in the other topic:
My personal taste aside (a lot of my favourite bands would be considered hard rock or even AOR by hardline "metal"-heads of post-thrash generation - which is my generation), I find it sad that there seems to be much less variety in modern "traditional" metal than it used to be. As one "Metal Archives" user wrote about the so-called NWOTHM: "[NWOTHM] is basically a narrow view of heavy/power metal as it sounded in the mid 80s. This has been true for nearly everything I've heard with the tag."
I find it quite sad, since nowadays it is much easier to listen to different bands and yet a lot of bands fit into what You have described as "those retro wannabes & outright imitators". I don't know if it is simply a lack of inspiration or fear of being rejected as "dad rock" bands by beforementioned hardliners if they incorporate some elements that are not considered "true metal" nowadays.
An indicative conversation I had with a very good friend of mine (who is also the only one that listens primarily to metal in our group of friends), who is slightly younger than me (26-27 years old at the time). He told me he considers early Judas Prieast to be "rock, not even hard rock, but just [classic] rock". I thought he was refering to Rocka Rolla and actually agreed with him, saying it "could be called 70s hard rock, but that I see his point". Turns out that he was refering to their EIGHTIES output (and I am not talking only Point of Entry or Turbo). From what I have heard from few other guys of the same generation, it seems that it is a common opinion.
When I see "NWOTHM", I can pretty much guess what it would sound for about 4 out of 5 bands. Although, it isn't actually that much about the style (de gustibus...), but the feeling I have that many of these bands play that style because it is expected from them as "traditional metal" bands and not because they don't want to implement other influences.
2. Lack of understanding what "traditional metal" actually means, leading both to lack of outside influences and to lack of appeal to fans of real early-80s heavy metal
Firstly, one note - I am NOT trying to force my opinion of what traditional metal is upon other people. I am quite aware that these opinions might vary from person to person. However, I still think there are some objective variables that should be taken into account when discussing "traditional metal".
What baffles me is that many people (especially those born in the late-80s and later, some with many years of listening to hard rock and metal music) seem to make the assumption "80s-influenced = traditional". Sorry, Death's "Scream Bloody Gore" and Mayhem's "Deathcrush" were released in 1987, Slayer and Kreator had 4 LPs in the 80s, Sodom and Destructiion 3 LPs, and hardcore punk was at its height at the time. It is generally accepted that a band can be classified as NWOTHM if it: a) has clean vocals; b) doesn't contain obvious influences from hardcore punk, hip-hop, alternative rock and other genres that were fused with metal and hard rock in 90s and later. This leads to speed (even melodic thrash), power and doom metal bands, which sound obviously mid/late-80s, being labelled as traditional metal.
I will again quote myself from the earlier topic:
Really, I am not oposed to bands playing heavy/speed metal or USPM or anything like that (I even enjoy some 80s bands whose style has been described as such), but find it a bit iritating when bands whose main influences are clearly albums such as Painkiller (a good album, btw) or late-80s epic metal bands, are being described as traditional early-80s/NWOBHM-like metal.
The word "traditional" can mean many thing, but in the context of metal music, it is usually used to describe NWOBHM and NWOBHM-sounding bands of the early-80s (and even late-70s), as well as those bands that could be classified as doom metal, but that sound similar to early (proto-)doom bands of 70s and early 80s. Although this might seem to be as restrictive as what I have stated in the first reason, the fact is that in the early-80s, heavy metal was not yet "codified" and the bands differed significanlty even within the same scene/country, not to mention between different countries. As other users have noted, these bands were not afraid to implement influences from blues, jazz, folk, prog rock, boogie rock, southern rock, etc, etc.
3. Singers sound too much alike
As much as many of us (especially those that have played an insturment at some stage of their lives and that at the same time suck at singing, me included) like to think otherwise, for most people it is the singer who is the most recognisable band member. Although I disagree with the opening post that "Vocalists sound too much like Bruce Dickinson half the time", I still think there is not enough variety in the singing styles of these frontmen. Also, it is extremely rare that I stumble upon a NWOTHM singer with instantly recognisable voice, while these used to be a must for almost every successful traditional heavy metal (and hard rock) band, few notable exceptions aside.
4. Focus on being able to play fast and not on actually writing good songs (NOTE: this doesn't apply ONLY to NWOTHM, but it affects NWOTHM)
Again, one disclaimer - I absolutely think that good (or at least competent) technical level of playing is a must (few really rare exceptions aside, haha). However, there is general focus, not only in NWOTHM, but in modern metal in general, to achieve the highest level of playing ability possible. As all people have limited time on their hands, this sometimes leads to musicians ignoring and not developing their song-writing ability. I know of A LOT of heavy metal and other rock guitarists (some of them personally) that are able to play faster than almost any early-80 heavy metal guitarist, but who managed (in the best case) to release few demos/"self-released albums" with very weak (but very technical) material and then needed to quit playing metal/rock music due to lack of commercial and critical success and now make their living as music teachers and backing musicians for generic pop/folk singers, and who haven't written a song of their own for a decade (not counting occasional 15-second guitar solos in pop and pop/folk songs).
5. Low expectations from the general local metal/rock community (NOTE: this doesn't apply ONLY to NWOTHM, but it affects NWOTHM)
The last (at least for now), but not the least, reason why I think NWOTHM is not reaching its full potential is general lack of expectations from the local public. Of course, this is something that I can only write from the perspective of local scene in my area, but it goes something like this (it will sound off-topic at first, but don't worry):
a) pop, folk, EDM, hip hop are the most popular types of music here, so there is general attitude that "we must support alternative scene at all cost".
b) This leads to anything not being clearly 2010s/2020s mainstream music being labelled as "alternative" and the term being used as a term of approval and not as a term of musical label. It leads to every local rock/metal/punk/etc band being suported by "fans" regardless of how terrible it is (and, trust me, I'll rather listen to a generic modern pop/hip hop artist than 95% of these bands, and I hate modern pop/hip hop).
c) As most of these "alternative" bands don't play metal, much less non-extreme metal and non-core genres, so any bands that sound at least a bit like traditional metal (including thrash, stoner, doom with clean vocals, even modern power metal) get praise from traditional metal fans in the area, leading to further lowering of already low standards.
d) As metal bands generally can't achieve real commercial success (earning enough money to make it a full-time job), these bands are satisfied by achieving their "local heroes" status and don't really do anything to improve their songwriting.
e) In the context of NWOTHM, this leads back to reason no.1 and the general lack of any variety or willingness to include other inluences in their music, partly due to fear of losing their "true metal" status