Season 1 was bogged down with a love triangle that failed to properly tie into the main story and ended up preventing proper development for both itself and the central story, leading to the horribly rushed finale.
The first half of season 2 had the same problem as season 1 dialed up even further. Korra's character at the start was terrible, the civil war plot wound up going practically nowhere and left barely any impact, the love triangle was still awful, more subplots that were completely unneeded and Unalaq was a total mess of motivations.
I'd argue the second half actually started picking up. The writing became more focused, Korra became level-headed, the love triangle was thankfully dropped, and the action scenes with Bolin saving the president and the siege on the southern portal were fun and exciting.
The whole thing goes to absolute shit once Unalaq fuses with Vaatu. After that point you get three asspulls in a row, a ridiculous kaiju battle complete with chest lasers, the loss of the past lives, and a ton of questions without any answers. If they just kept the whole battle as Korra vs. Vaatu without any monster transformations, it would have been good.
Season 3 is good. They clearly learned from their past mistakes, both the subplot and the central story have clear arcs and tie together nicely, the cast is at the very least decent and everyone gets nice moments, great action scenes, fantastic villain, colorful and interesting locations all leading up to a finale that I consider the best in the franchise.
Season 4 isn't as bad as 1 or 2 but suffers from a massive pacing issue. If it was intended to be written as tightly as 3 was, getting an episode cut hurt the season badly. It was off to a great start but meandered everywhere, focusing on side characters that didn't need so much focus and neglecting to build the story until the last couple of episodes arrived and we got saddled with a giant mecha.