I watched the entire show from beginning to finish, and was a 'fan'. Not a hardcore-blinkers-on fan - I've criticised plenty of things about the show - but someone who just liked the initial concept and stood by the show until the very end because ultimately, it was still a show about my favourite comic book character.
I liked the casting.
I liked the rich history they gave some of the characters.
I liked the stories which were, for the most part, good - although there were a few duds along the way.
I liked the effects, which were superb in the first few years for a TV show, though they did suffer a bit as budgets were reduced.
I liked the characters - Superman was only one part of the Smallville mythos, and we got to meet many of his DC stablemates such as Flash, Hawkman, Green Arrow, etc. Batman was the most obvious absence but I guess that was just too much to ask.
I liked the show's determination to stick to telling the story of Clark Kent, and not to suddenly become something it was never intended to be (a Superman TV show) along the way ... hence the no tights/no flights rule.
I liked the cameos from Superman-related actors - Christopher Reeve, Margot Kidder, etc.
There were of course things that weren't popular.
I didn't like the style of writing, which was a typical CW trait. Too much Dawnson's Creek-style prose, cliches, ridiculous narratives and so on.
I didn't like the the handling of some key villains. Doomsday for example - the encounter between him and Clark was a huge anticlimax. Darkseid was also a disappointment - huge physical presence, but they chose to have him mentally inhabit a human. Again this was likely due to budget constraints - understandable but nonetheless disappointing.
I didn't like the back-and-forth and constant will they/won't they questions between Clark and Lana. They dragged that one out waaaaaaaaaaaay too long - again, another typical trait of CW dramas who seem to think it injects tensions into a show. It doesn't.
I didn't like the budget constraints in later seasons. It showed. The set they used for Metropolis (and any other street shots) in the last few seasons was woefully overused. Even the extras seemed to dwindle away to nothing - large crowds on the street in the first few seasons, to 3 or 4 odd spectators in later ones.
I didn't like some of the iffy acting. Wasn't even the main cast usually, but random actors appearing as a one-off.
And like many viewers, I didn't like that we never got the money shot - a full view of Tom Welling in the suit. They teased it, we got some partial body shots and distant CGi, but never a full Superman pose. I understand the show was always about Clark Kent becoming Superman, but nevertheless the finale was the end of the road - Smallville had finally arrived at its destination after a 10yr journey. I felt they owed it to the show AND the fans to proudly present Superman, showing that Clark had made it and what all his trials and tribulations had achieved. It was teased instead.
So all in all, I liked the show a lot but it had its flaws. And I guess that's why it still polarizes opinion to this day; how much you like the show was largely dictated by how much you were able to see past its flaws. Some people just couldn't buy into the teenage romance angles, the writing, the constant use of meteor rock as a plot device.
Despite the flaws, I felt the show tried hard, and for that reason I stuck with it.