Smallville seems to have a couple different of audiences. Many fans who actually did like the early conception of the show probably did not like its later years. Particularly fans of characters/actors who left the show.
I never really liked the early concept of the show. While I think it made improvements later it was still tied to its early history. But also like most long running show the later seasons seemed really low budget. So while the ideas became more ambitious they did not have the money to show them. It was going to the cast. Which always happens, the longer the run the higher the salaries are to keep the original actors.
Looking back now, the production values seem lavish on the early seasons. Not just do to more money but they were working within their limits. The production crew did not have the experience to really pull of the demands of the later years. Watching the old Superboy show is interesting in contrast to Smallville. VERY LOW BUDGET. But at least they had more effective shot physical stunts and the flying wire rigging. Without the gloss of modern CGI which Smallville used to dress up its really dull fight choreography.