I think Season 2 is so poorly received because there's no immediate threat? The main conflict for the first half of S2 is looking for Sophia and let's be serious, the possibility of an abused mother losing the last part of her family is dark and awful and I could never fathom what Carol had to go through.
That said, it's not 'threatening'. Especially as Sophia barely spoke and most of us had no real attachment to her. And besides, what were the outcomes? She lives, the group remains as normal. She dies, they will mourn and they'll carry on as if one of their own had died. Essentially, the first half of S2 had no threat, which I doubt people didn't enjoy from an apocalyptic show.
Then the second half came along and I'd argue Shane became more and more the 'villain' post-Sophia's death. But at the same time, it wasn't as if he was a threat either? He was just super pissed off at Rick, but I highly doubt he would put his group at risk. He was just volatile and developed a mindset and view of the world that the group would later develop in a few seasons. I'd honestly argue that Lizzie was a better 'villain' than Shane (even though Shane was a massive dick and was awful to Lori once Rick returned, these never contributed to how threatening he was to the group, whereas Lizzie was willing to
kill her own sister and a baby).
Of course, S2 did develop a threat in the finale with the massive horde, and it made for, what I argue is the best Season finale of TWD. But that was only for one episode
And then you compare that to the other seasons.
Spoiler:
Yes, Season 1 had no villain, but the threat of walkers, the camp being attacked, and a purpose (the CDC) all contributed to a drama Season 2 never had.
Season 3 had the Governor, the return of Merle, the realisation that there were other communities. Plus a lot of death.
Season 4, again had the Governor as a threat, and the virus in 4a. A lot of people seem to dislike 4b and that's when those two elements were taken out. With no threat and rather focussing on the group and their relationships, I would compare 4b to Season 2, and that's why both are generally disliked. Yes Terminus was a bad place but we found that out at the end
5a had the mystery of Beth's location, and 5b had finding Alexandria. Can't forget that this season did have a few deaths too.
6a had the horde, resulting in the best MSP of the series so far (No Way Out :love) while 6b established how big the world really was, and the Saviors.
Season 7 was pretty much about the repercussions of the season 6 finale, which was enough to fuel the Season
and 8a has had the war going on