What went wrong with Battlestar Galactica?

I'm not the only dissatisfied viewer of the late Battlestar Galactica re-imagining; I've found some complaints across the net. It's a huge disappointment for me; I really liked the show's early direction. However, I haven't seen anyone complain about what really bugged me, so I decided to collect my thoughts and see if anyone could confirm or (more likely) correct me.

• The biggest offense: The Plan. The series promised early that the Cylons had some kind of plan. At one point you could even guess one of several things: that they were pursuing the few remaining humans of the Twelve Colonies in order to find the humans of the Thirteenth Colony in order to wipe them out as well; that they wanted to create of a hybrid human-Cylon race (see Baltar's vision, below), or even that they wanted to keep a farm of humans for use as hybrids. It could even have been any combination of the three, or something else entirely that at least would have made sense, and I would have been satisfied.
  
  The writers were not, apparently, satisfied with any of the explanations that would have made sense. They never did explain it during the course of the series, but they couldn't just pretend a plan never existed (lots of questioning about this in online forums), especially since opening titles of the show's first seasons included the announcement, And [the Cylons] have a plan. I wonder how many times the writers sat around regretting that. Whatever the case, they scripted a special film, post-conclusion of the series, to explain the plan. It turns out that… "the plan" was not the Cylons' plan; it was one Cylon's plan: the Cavil model's plan, to wipe out the human race so the universe could bask in, uhm, "justice".
  
  That doesn't fly. The opening titles of early seasons alluded to a plan in the present tense. The "present" here indicates the time after the near-elimination of the human race. I don't buy it.
  
  The one mitigating aspect of that otherwise forgettable film was Dean Stockwell. BSG has at least featured many great actors: Olmos, Stockwell, McDonnell…
• The revelation of the Five Final Cylons was so unbelievable, and the method used to introduce them so ridiculous—a Bob Dylan song playing in their head! really!!! a Bob Dylan song!!!—that I was sure the writers were merely introducing a new Cylon tactic: what marketers refer to as FUD (Fear, Uncertainty, Dismay). That would have been all kinds of awesome, and could have redeemed and reinvigorated the show's clear decline throughout its third season. Instead, the writers doubled down on this bet.
  
  Again, I don't buy it. A flashback early in the series shows Tigh and Adama meeting as a younger men. Tigh is both balding (as opposed to bald, later in the series) and has hair color (later a stately white). This is a problem because Cylons don't age. Cylons whose bodies are old, be they Cavil or Tigh, are born old. Tigh's body is shown in a Resurrection Ship, and he looks old. Indeed, if Cylons could be born young, it would have been stupid to infiltrate the fleet with copies that are all the same age.
  
  Then again, as the third and fourth seasons progress, the Cylons are made out to be increasingly stupid, emotional, and petty, flatly contradicting earlier seasons of the show, where they are portrayed as brilliant, logical to the point of exploiting human emotion, and profound.
• The meaning of Cylon "evolution" changed from the beginning to the end of the show. At the beginning of the show, it was clearly implied that the biomechanical Cylons were an evolution of the purely mechanical kind. At the end of the show, the meaning was completely different, because the very word "Cylon" had assumed a second meaning. By the end of the show, "humanoid" Cylons were not, in fact, an evolution of the "toasters", but were instead an invention of the misnamed "Final Five", misnamed, I say, because they were, in fact, the "Original Five" humanoid Cylons, and weren't even mechanical originally, but were human, only not really human, because they were from the mythical Thirteenth Colony, which turns out to be a Cylon colony, not a human colony—Because the Scriptures of the Twelve Colonies would have mentioned a Thirteenth Tribe that went in search of another planet and was never heard from again, but would never have mentioned that this thirteenth tribe consisted of enemies of the other twelve, let alone non-human beings.
• Gaius Baltar's second, third, and fourth acts ruined a really good character. At the outset, Baltar is a scientist who is trying to hide the fact that he accidentally betrayed the human race to a Cylon of whom he now has recurring visions. He more or less abandons science all too quickly, at first for politics, in a way that is almost believable, and subsequently becomes a religious leader. Most disappointing to me, Baltar experiences at the end of the first season a vision regarding a child that he supposedly must guard. This is how Battlestar Wiki describes it:

  Entering [the Opera House], he is shown the "face of things to come", apparently a baby in a crib, the "first of the new generation of God's children" - and he is to be their protector.

  This never materializes, but a different vision later in the show, shared by several characters, tries to supersede it. Baltar never assumed the role of protector for anyone, let alone the the "new generation of God's children".

Oh, well. It's only a TV show.