In terms of Dexter's growth, I think we saw growth in Season 1 and 2, and to a lesser extent in Season 4. In Seasons 3, 5, and 6 we've seen hints of it followed by hitting the reset button.
Season 5 seemed to have Dexter really coming to accept and even LIKE himself because Lumen cared for him and he finally found someone he could connect to....and then she bailed. Season 3 he thought he could connect to Miguel and then WHOOPS not so much. Season 6 introduced faith and Dexter's questioning whether he could truly reform himself....JUST KIDDING!! Well why freaking introduce this stuff if you're just going to undo it in one episode?! And really, what has he LEARNED from any of this?
You'd figure after Season 2, he'd isolate himself more or blend in more with his "cover" persona, but no, he tries to find a "buddy" in Season 3. Then, oops! We have to get rid of him, but wait -- he still hasn't learned because NOW he's looking for a TUTOR in the form of Trinity. Or at least as an exemplar for how to live a balanced life, except -- oops! Now part of your family is six feet under (see what I did there?

) In season 5 he goes back to this same "I need to find someone I can connect with" thing and....yoink! She leaves. Now in season 6 we have him maybe thinking he can reform himself, but, naaaah, never mind that. And then maybe he can reform Travis until -- sorry! -- he's actually just nuts.
In some cases, these things get introduced as a necessary factor in the season's story arc...but it never really seems to register for Dexter. He's the same guy the whole way through. And it's not that this stuff isn't entertaining (usually), it's just that it's LAZY. I mean, jeez, I've seen sitcoms with better character development than how they treat Dexter.
My complaints about other seasons were different. Mostly surrounding the worn-out plot point of "WILL HE GET CAUGHT?!" That's one thing I've LOVED about this season -- none of that crap going on. There's been zero danger of Dexter getting caught this season and I LIKE it that way mostly because the previous seasons went back to that well FAR too often and I no longer give a damn when they try that tactic. Other problems I had were with the introduction and then abrupt dropping of plot points. Remember the female cop Batista got involved with? How about the Santa Muerte stuff? And what about Deb digging into her past and finding out that Dexter's mom was her Dad's informant? That got dropped like a bad habit. Batista marrying Laguerta out of the blue -- and then the show retconning that between seasons too? That stuff was all filler. All of it. It served no purpose other than to take up time, and that, to me, is lazy writing. The show is -- or was at least -- capable of far, far better.
As for the next seasons, I don't know. I don't think I really care much, either. I mean, ok, he gets hunted by Louis and Deb finds out somewhere along the lines. More likely, they'll just do yet more of the "WILL HE GET CAUGHT?!" crap that I've grown tired of and I'll finally tune out altogether. Or, maybe it'll be fun -- like this season -- and I'll hang in just to see how he'll get out of this one.
But you know what? Dexter has become, for me, a lot like Star Trek or SG-1 -- it's a fun show that I really enjoy, but I don't take it seriously because the stakes are never really that high, and because the characters, while entertaining to watch, never really seem to change. I can still absolutely enjoy the show, but I can't take it that seriously.
The only characters I've REALLY liked this season are Deb and Quinn, mostly because of how they've changed on the show. But hey, who knows. Maybe that'll all be retconned by next season, too.