From the National Conference of State Legislatures blog, some data about who's running for state legislatures and who's not.
There are at least 6,129 legislative seats up for election this year. There are 10,728 candidates running for those seats under the banner of 38 different party designations. The Republican party is fielding 4,795 candidates and the Democratic party has 5,135 entrants. There are 798 people running who are affiliated with 3rd parties--mostly independents, greens and libertarians but one guy in Vermont who represents the "Wingnut" party.According to [Professor John McGlennon and research assistant Cory Kaufman at the Thomas Jefferson public policy school at the College of William and Mary], there are 2,175 candidates running without opposition from the other major party (NCSL's numbers are slightly higher but close). That number is roughly in line with the trend since the 1998 election but higher than the early 90s.
What is interesting is that the Democrats are running without GOP opposition in 58% of the unopposed races, but Republicans are unopposed in only 42% of all contests. It could be a sign that the Democrats are energized this cycle and fielding a large number of candidates hoping for a wave to sweep them into office. Or it could mean that Republicans are more strategic by only devoting resources to races where they have a legitimate chance to win.