I'd have to disagree it's a matter of pacing preference with Season 1. The Fafner franchise, of which I've become a big fan, knows how to split cool-down and development time with fighting. Exodus, the sequel to the original Fafner, had some pacing issues near its end. But even so it still managed to combine downtime with showtime. The first 13 episodes of Exodus were perfect, as good as it gets, giving whole episodes to show how things were "taking root" as you mention Phillosmaster. But that didn't stop the series from having energy! Far from it, in fact the series brims with desperation and uncertainty.
IBO had a low-key setup that reminded me of Cowboy Beebop with a smudge of Outlaw Star, focusing on a down in the dirt story with young men trying to find a better place in the world, some of them with typical philandering bachelor personalities and outlooks on the world, some who desired a place to belong, and an almost mysterious yearning from Orga and Mika to find their "place".
But neither of the previous mentioned shows felt slow or off-beat the way IBO's S1 did. They built significant plot up and finished it entirely by their final episodes. Other Gundam series have had significant time for characters to just sit back and coast. Zeta had plenty of this, even with a battle in almost every episode. There was always time for a philosophical debate or discussion on the nature of the soul or state of the Federation's union.

IBO, by contrast, spent a great deal of time not really moving at all. There were plenty of cases where characters didn't actually say anything of worth that had not already been repeated numerous times. This is why some argue that the characters didn't really develop. They mean the IBO boys didn't really grow out of who they already were. Only Kudelia, Biscuit, Fumitan, Orga and Mika (in that order, imo) expand beyond their skeletal structure. Two of these three parish by S1, and Kudelia disappears as a primary element in S2, despite being relevant anyway, which I think is testament to her growth.
Eugene's big scene, for instance, was a nosebleed and ramming a ship into a war station, something not really impressive compared to earlier feats on his part... Poor Akahiro doesn't really gain much or show much onscreen, even after the cheesy loss of Masahiro, aside from making a nice statement. And then he returns to the gym and we hear no more from him outside of combat.
But really, to undermine my whole lengthy babble up above, the characters developed fine, and I argued for them all through S1. But what really goes wrong is how slowly this happens. It's like taking too little paint and spreading it over too much wall, to rob Tolkien of a useful metaphor, revised.

There was an article, I think LightningCount posted it, that had the director and Okada admitting they really didn't have an exact plot laid down for the first half of the show outside of "escort Kudelia to Earth." That kind of plot doesn't require much time, especially if it lacks significant hurdles. The worst we got was "it's a long way from Mars to Earth"... right. But not so cool when much the time spent on this journey involves the "Dead space". This doesn't include necessarily the "laid back" talking scenes, but waste of actual airtime in how the episodes as a whole were structured. The dialogue sessions especially suffer from the overstretched structure, which is why many point out to them as the cause for the snail's pace, especially with little enough action to be had.
We have this problem, so far as I've been able to tell, rampant in most modern anime. Code Geass is an interesting exception that parallels well with 00, which ran around the same time. The former doesn't have this issue, it's plot never or rarely allowed for it.
Conversely, 00 wastes lengthy minutes on conversations that could be happening much faster, complete with long moments of "thoughtful pause" in which we hear no internal dialogues or psychological dissection to speak of. As far as I care this is indeed "dead space". Unlike 00, which advanced its plot at a brisk pace throughout its first season, and even into S2, IBO turtles through each episode in this fashion (the paint on the wall again), as if it knows where it's headed but doesn't feel especially happy to get there any time soon.
S2 by its half-way mark already gave us significant advances in plot, character growth and development. I believe the main cause of this concise narrative structure is a direct result of Okada, the writing staff in general, or even the director, knowing exactly where they plan to take things and how to get there. Maybe that will prove untrue for the remaining episodes. But at least S2 thus far hasn't been slow or meandering.
One could argue that, maybe, knowing they had a second season and needed a "stopping point", the staff decided to draw out S1's plot instead of thickening it. This doesn't require more fights, but could have at least given us the wheeling and dealing of S2 behind the scenes. Instead, Nobliss doesn't pop up much, the Old Man snips a couple banzai limbs, and the mysterious Makanai is totally absent until they meet him at last, and then spends a couple episodes babbling over what could have been said much, much sooner had the plot allowed for it. The Gjallarhorn leadership is entirely absent, and if I had to pick one thing only to change, it would be this.
Say what we will about Iok "Idiok" Kujan, but his character engages us into the story, if in a way villains are supposed to "make us angry". Galileo's father held prominence in S1 for the whole loli-marriage arrangement, and yet isn't shown until S2. Most notably, Rustal Elion is nowhere, not even given a name drop, despite being responsible for the Dort fiasco, or at least part of it.
It's nice to get slice-of-life or pleasant, non-life threatening Bebop drifting now and again, but IBO did not pull this off successfully at all, and instead simply bled enough potential for fans to scream for more while we had the chance. We've gotten more, and so far it's been worth it. But many wish it had been worth it sooner, and not sacrificed nearly half its 50~ episodes to do it.