I've watch season 1 - season 8 in just under a week, and I watched the whole of season 6 in one day, one after the other, that hurt might I add... and I'm grabbing season 9 now...
I originally stopped watching around season 7, but don't recall alot of season 6...
Onto what Smallville lost...
Lex being gone, a big let down, he was a character I watched for.
Lionel being gone, a huge let down, in the last two seasons he was in it I really enjoyed his portrayal.
Lana being gone, then coming back, then being gone again, annoying. So damned annoying.
Kara's in and out nature, kinda annoying.
I'm not a comic book fan, so the almost episodic justice league references were distracting, since they were aimed at those people
Fundamentally, yes, it does get worse after season 4, is ok in seasons 5 & 6, seasons 7 and 8 were sorta tedious given the erratic storyline (to me it came across as erratic) and the time travels episodes, but the completionist in me dictates I have to see all of it since the show is ending at season 10
As for the month to month episodes, I sure as hell hope they don't do this to the final season, I know there will be a christmas hiatus, but hopefully not the highly annoying 5 month ones that they pull on some shows (ending in October coming back in late February early march) and yes, its just to drag out the show or to try and build suspense, but it actually has the opposite effect on me, I stop watching, like two shows recently, they were off the air for 4 months and I never returned to them.
Now I ask myself, should I have dredged up a 7 month old thread?