Well, no, this sort of thing is always a matter of taste. I can see your point. For me though, in a subtle way there is a fair amount of character development here. Part is from people's actions and reactions. What a person decides to do has always been a good tool for showing character, but it doesn't work if there is no character to reveal and so they react just like every other. Here I do feel that there are gradations of Suiren's decisions that give dimension to her quietness. Aside from that, for me the art itself is so good that it reveals a lot more about the characters' feelings and thoughts than a typical mangaka could; I feel like I'm absorbing an understanding of Suiren, for instance, just from subtle shifts in expression.
In real life there are quiet people who do have personalities and it's hard to point to just how you figure out what they're like, but you do. Portraying something like that is I think very hard to pull off in manga; at any rate it's very rarely attempted. So I like that, find it interesting.
Plus, I'm a sucker for very pure vanilla romance. I actually like that this kind of goes back to the roots of shoujo romance and tells the simplest story--no cynicism, no triangles, no weird plot complications, no bizarre character backgrounds, just two people and romantic feelings. Again, this simplicity is hard to pull off because it leaves just the bare characters and their simple interactions carrying the story; if you don't do it very well indeed that won't be enough. In this case, for you it wasn't enough.
One could argue that I'm handwaving furiously here and the basic reason I'm still reading this is I just can't get enough gorgeous art of Suiren looking ludicrously sweet and cute. There would probably be some truth to such a charge, but I do think there's something to what I've been saying.