NOTE: I know much of this has already been said, but I really felt the need to get my thoughts in writing. xP
Dan's first major mistake was promoting Gibby to a main character. Gibby was awesome back when he showed up at random times and had funny one-liners. However, all that changed as soon as he was added to the main cast. Suddenly, his character wasn't as fun and loveable anymore, and instead was conceited, obsessive, stupid and an overall idiot. Not to mention that Dan created the absolute lamest catchphrase for him (Gibbeehh) and he decided to stick with it like hell. Plus, the fact that he was older now made his role a bit more awkward. Perhaps the saddest part of all is that he no longer took off his shirt. In my opinion, his addition was probably the worst part about Season 4.
Shorter Seasons and Seriously Long Waits Between Episodes
Okay, I suppose this is more where Nick went wrong. Starting with Season 4, the seasons became painfully short. I believe this has a lot to do with Dan's trasition of focus to Victorious around this time. To make matters worse, there was an unbearably long wait between episodes. I'm sure you all remember the insane dry-spell we had while waiting for iHire an Idiot. And for what? The episode didn't even end up being that good. A lot of long-time fans of the show began to steadily lose interest around the time this was happening. Who could blame them? It quite often wasn't worth the wait.
iStart a Fanwar
This one's an iffy one. While the episode itself was horrible, it did (sort of) make a good point that some of us were taking the shipping on the show a little too seriously. Had we known how the series was actually going to end, we would've taken this advice less lightly. However, it still didn't make sense to air the arc almost directly afterward. All that does is alienate and confuse the viewers.
The Seddie Arc
Here's where I have some real beef. First and foremost, I think it was completely unnecessary to dedicate 5 episodes to Seddie. In my mind, I had always envisioned either them dating in the background of normal episodes or just hooking up at the end of the series. iOMG and iLost My Mind were both great, but it all went downhill from there. Apallingly, Carly's character was completely neglected in iDate Sam and Freddie and iCan't Take It, and her character continued to lose importance for the rest of the series. Considering she is the main character and she suffered a severe lack of development, it really does say something. Anyway, back to the arc. Sam and Freddie were NOT acting like the Sam and Freddie we all knew and fell in love with, and instead were being completely immature and childish. Seddie was just kind of ruined for me and for a lot of other Seddiers with this arc, because it didn't emulate the perfection that was Season 3 Seddie. That being said, I was happy when they broke up in iLove You. Everything about that elevator scene seemed to indicate that Seddie was going to get back together at some point. Unfortunately, this did not happen.
Seemingly Disinterested Cast (aside from Jerry xP)
Something I could not help but notice in the later seasons was that the cast members seemed to be a little less enthusiastic about their roles. In particular, Jennette seemed to play a more toned down Sam and Nathan played a more "macho" Freddie. In other words, they all seemed to be playing themselves rather than their characters. I couldn't have been the only one that noticed this. Though I could be wrong, I feel like this really distorted the chemistry of the iCarly trio, and we were already getting less and less Creddiam episodes.
Lack of Proper Resolution of the Ships and Reverse Character Development
This is probably everyone's biggest complaint. Freddie's crush, something that hasn't been mentioned or visited since iSaved Your Life, was suddenly ressurected in iOpen A Restaurant. This would've been fine if they actually stuck with it and Carly and Freddie actually has a serious talk about the "hero theory" and getting back together. Instead, they made Carly desperate for boys that weren't Freddie. To have Carly suddenly have feelings for Freddie at literally the very last moment was just a little too unrealistic for me. As for Seddie, despite the fact that iLove You ended so ambiguously, they never had the clousure they deserved either. All they got was a phone-call that made Freddie's feelings even more vague. If Dan didn't really care about the ships and only put these scenes in to please the fans, he shouldn't have had any shipping moments in the finale at all.
Sam and Cat
This idea is incredibly uncreative and unoriginal. Jennette and Ariana need to move on from Nick and acquire more mature roles. That is all I'm going to say about that.
Ah, the Glory Days...
Perhaps Season 3 was simply too amazingly perfect for the rest of it to live up to. Although, it was arguably starting to lose it's way towards the end with episodes like iWon't Cancel the Show and iBeat the Heat. Still, how hard would it have been to just end the show there before it went downhill...? :/
Okay, depressing rant over! :DDD