Frank Darabont is really good at telling specific types of stories at a specific scale, which season 1 fit really well for. You could constantly tell that the story was being driven by his personal interest and care in it (he even brought some of his acting regulars over).
Even just by season 2, it had firmly pivoted off its success to a show intentionally built to be a tentpole long-term investment for AMC. A show of that sort requires more committee level decision-making, and safer, less personal storytelling designed for long-term planning. The show moved away from the tight narrative centered on Rick and Shane to work with more sustainable ensemble style narratives. New characters were clearly framed to be fan-favorites. Deaths and big events were scripted and paced out to serve largely as chatter-generating events.
They did a really good job making the show what they needed it to be—it's become the network's most successful tentpole (by FAR) for a reason—and it's perfectly fine for a show to operate more broadly as it gains popularity. But no one really seems to talk about it as the underlying cause for the most common complaints I've seen (repetitive season arcs + loss of grounded central narrative).
Personally I felt like a lot of my time was being wasted in season 2, and the first parts of season 3 turned me off from the show for probably similar reasons (don't quite remember), but I do think it's an interesting piece of media that's worth discussing :)