Like someone else in another thread said, they basically wanted to make TNG/VOY in the 22nd century.
Rather than make a show appropriate to the times in which it was set.
Some of the scripts were actually quite good.
But, they paradoxically wanted to be free from canon constraints - to make a quiet family show - but chose a canon-rich era, known to contain vast political change (the formation of the United Federation of Planets, the start of the Earth-Romulan War) - which is quite the unreasonable expectation, given the difference between desire and reality. It also came off as quite self-entitled. They wanted that era because they wanted to make ENT more like "The Right Stuff", chronicling early spaceflight - we can debate whether they succeeded at that or not, because the show sometimes really did provide this feeling - episodes like "First Flight" tried to capture that Chuck Yaeger/Alan Shepard type attitude - but a lot of people felt they could have gone further in making the era more primitive.
"There is no starship mission more dangerous than that of first contact... centuries ago, disastrous contact with the Klingon Empire led to decades of war..." - Jean Luc Picard
Right off the bat, they alienated a lot of fans by contradicting things that had been accepted as beloved parts of Star Trek's history, for no real plot reason (it would have literally been just as easy not to) - the content of the Star Trek: Encyclopedia and Star Trek: Chronology were ignored - both of them had been scrupulously written from on-screen evidence, using a minimum of speculation - partly to actually aid writers, who could have just flicked to the relevant page, and learned what they needed in seconds.
One example was a Klingon landing on Earth in Broken Bow - but first contact not being 'disastrous'. Not having the Romulans, Star Trek's first enemy ('Balance of Terror' being older than 'Errand of Mercy'), turn up before the Klingons, was a bit of a slap in the face to their vintage credentials - but wasn't technically an error, just a re-interpretation. There was the decision to have a K'tinga-class ship show up 100 years early just because the producers wanted 'more windows' on the painstakingly built model they would later use, after someone had basically worked nights to prepare it in their own time (honestly, w. t. absolute f.?) But, of course, the absolute daddy of them all - the undisputed worst continuity error in Star Trek's entire history - the Romulan cloaking device needlessly (utterly needlessly), showing up in an episode that did not even require a cloaking ship - contradicting one of Star Trek's all time greatest episodes, "Balance of Terror" - which the producers would have known if they had even bothered to watch TOS's top five episodes, let alone all of TOS.
And the episode itself? Despite "Minefield" being an utterly fascinating concept, it is, somehow, actually boring to watch. Yep - first contact with the Romulans - a character piece about a captain risking his life for his tactical officer - a ship striking a mine in space - was somehow made boring (that actually takes skill). Which is where ENT's real problems were - it's quality of writing.
If they wanted a blank canvas, they should not have chosen the 22nd century - if they wanted to chronicle early human spaceflight they (perhaps) should have embraced it more fully (although it actually works in hindsight) - they (perhaps) should have engaged with The Romulan War more fully if they were going to set a show on it's very doorstep, but wanted to make a peaceful show, not a war show (again, dissonance between desire and reality). If they had made the NX-01 genuinely primitive like an Earth alliance ship from Babylon 5, complete with huge fusion engines and rotating centrifugal habitats - done a show in which every spacewalk was a tense moment - it might have worked really well and been remembered as something special in Star Trek's history. As it happens, having a recognizably Starfleet-like ship works alright too, and cements the show's identity like a brand - but still, at the time, it seemed unambitious when Babylon 5, for example, had done it so well.
Discovery really does not suffer from any of these problems - it's not set at the beginning of spaceflight - it's set in an era in which nothing earth shattering is known to have happened - the only historical context to the era, is the Klingon-Federation Cold War - the only constraints in technology are to not appear too different to TOS/TMP, which is a standard that is actually quite open to interpretation, since we don't really even know what technology was available in Kirk's era. It will allow for highly detailed sets, EVA missions, hull breaches, etc. Also, this time, it is actually being written by someone who has, you know, actually watched TOS. Plus production design teams tend to be really well-researched these days, so I would guess they are doing everything imaginable to make a smart design - the first evidence of this being the selection of a really interesting and appropriate ship.
In hindsight, ENT is a lot stronger than people remember.
But it still requires you to actively forget the missed potential, which is painful to remember.
Discovery has nothing to live up to - it's set in an era that is quiet in terms of known events.
The theme of early NASA-like pioneers was actually a good idea, and could have been done well - but I'm not sure it was really a good idea to do a fourth TV show with a tired team - who basically pitched every episode as if it was a Voyager episode? Even Season 1, and Season 2, are stronger than people remember, especially if you watch them forgetting that the show could have gone for a far more formative feel. Season 3 is as good a drama as Battlestar Galactica in places. Season 4 attempted to tie the show into the wider Star Trek context and succeeded somewhat, but perhaps cut the formation of the Federation too close - so that the Andorians went from war with Vulcan, to binding sovereign union, in a handful of years.
Of course, this post just covers canon and continuity - what many people actually felt was the nail in the coffin for ENT was it's quality of writing - too many forehead-of-the-week episodes with no real hook - great concepts made somehow mundane and boring.
