Personally I still really love the show, but I do understand why people dislike it. Firstly, zombie fatigue. Secondly, they occasionally make really dumb decisions- there have been several times where I've been left thinking 'Carl, why are you such a twat?' and sometimes it can be a bit slow. The second and third series do lack a lot of action mid-season, but the finales are always worth it.
And yeah, some of the actors suck a bit, but I think the most important roles are well done- Rick, Daryl, Shane and the Governor were/are great (although David Morrissey is never bad in anything). Lori, Micchone and Carl, not so much.
From what I can tell from the Season 4 trailer it looks a bit more action orientated, although that may well just be edited like that. Showed more raiding for supplies, which they never really did in season 3 (or, when they did, they got sidetracked). Being stuck in one place really hurt the show by taking some tension out I think, that's part of why season 1 was better.
Someone already mentioned that it's not the best shot show ever, and I'd agree for a big budget US show. Everything looks a bit bland, flat and colourless, but I suppose it is meant to be a zombie apocalypse in Georgia so I see what they're going for. On the plus side the music choices are usually very good: