Back in '79 I enjoyed TMP overall even though I (like others) had reservations. However, I wasn't of the mindset that it was a failure. I saw it as a near miss. I thought it was good and wouldn't have required much to be better. But there were others who just wrote it off as a huge disappointment.
This certainly wasn't the only time I'd see this over the years. It seems to be conventional wisdom that Spider-Man 3
and Iron Man 2
are crap. They're actually not bad at all, but they're not as good as their predecessors. But the sentiment seems to be if it's not awesome then it's plain crap, it's a fail.
I see only a few missteps in TMP, and here I'm talking about the film in general and not specific versions. The DE fixes some of the missteps of the previous versions simply because those were easy to fix, things that would have been fixed in post production given sufficient time.
In terms of story there are are two basic missteps: there needed to be more character drama and the ending wasn't big enough. The ending of TMP is similar to the ending of TNG's "The Best Of Both Worlds." Both are logical resolutions in context with the events leading up to them, but each are wanting dramatically.
In TMP's case I think the dramatic missteps could have been fixed with some deft rewriting. The story as is works well enough if we're talking about a novel, a work in prose, but it doesn't fly as well onscreen. It needs more dramatic juice. TMP as is works better if you can pay attention to a lot of other things going on in the film, but if you just sit there passively waiting to be stimulated then it might not make an impression on you.
Visually there is another miscue. I don't mind the TMP uniforms in general, but that said I think they could have done better. I've seen the uniforms up close on display at an exhibit, and in person they're really nice. Unfortunately it didn't translate as well onscreen. Something a little crisper and with a bit more pop would have been better. I think it also would have helped counter the complaint that the film seemed rather monotone. I think they could have found a way to inject a bit more colour into the designs as a nod to TOS without going too bright and the designs could have been crisper.
Just an idea I photoshopped some years ago.
For myself I would have paralleled TOS a little more in concept. Command could have been something of a taupe-ish gold or even a soft greyish brown. Sciences could have been a soft blue. Medical a soft green (as pictured above). Engineering and support services could have been grey and Security could have been a darker grey or grey/blue. You could even retain the concept of the whole uniform being the same colour rather than department colour over black trousers as in TOS. I would have also kept the insignia more consistent with what we saw on TOS. I like the retaining of the rank braiding on the sleeves. Something like what I'm suggesting would have seemed more like evolution rather than confusing wholesale change. In TMP department was denoted by colour in the circle around the insignia. Well thats fine in the real world, but it's too small a detail show up clearly onscreen. Department denoted by uniform colour is a lot easier to see and understand onscreen.