Leading up to this season, Smallville was more "earthbound," in that it really was about a teenager with developing superpowers with only mere glimpses into his true heritage.
Season Five changed the tone of the series forever.
I always viewed this season as the mythology turning point for the series. Huge things happen and we're suddenly thrust from those Twilight Zone-oriented episodes into the DC Comics Universe; things shifted from a "superboy" to Superman.
Significant events go down this season, including the emergence of a certain location and the death of a certain loved one. We're also introduced to a couple more characters from DC Comics continuity.
For me, this was the season where the show grew up and strolled down the Superman road with purpose instead of by happenstance. (Those who've seen the show know what I mean.)
The only thing that irked me about this season and going into the next was the whole Clark and Lana thing was getting played out and it seemed the romantic subplot of the show was going in circles instead of officially stating, "Let's get these guys together for real and keep it that way." But, as per usual TV show fashion, something always has to happen to drive the couple apart. Though romantic tension is good up to a point, Smallville set the bar high in terms of driving the audience nuts--even to the point of bad-TV frustration--with the Clark and Lana relationship. Sometimes we as viewers just want to see things work out, you know?
Other than that, good times to be had in Season Five of Smallville.