Every season in The Wire is a mix of new and old groups of characters (except the first of course). Is s2 really notably extra focused on s1 characters? If it is I didn't notice, though I can't say I've counted either.
Season 2 helps explain how the drugs come into the city. It shows how the cops get reassigned to hammer on which ever nail is currently sticking its head up once they've hammered the current one back down. It shows how nails can work their way free and pop their heads back up requiring the attention of the hammer again. If only the cops could figure out how to use screws instead.