In the 2016 election, only four Republicans won state House seats with no opposition at all; three had Libertarian or Green opponents, making a total of seven for which the Democrats were no-shows. Getting that down to two in 2018 may mean some progress for the Blue team.
There will be a few contested primaries, including three-way races for Districts 133 and 146. The latter district is currently represented by Shawn Thierry, chosen by local precinct chairs to replace now-Senator Borris Miles, who replaced now–County Commissioner Rodney Ellis, who replaced the late El Franco Lee on Commissioners Court.
Quantity isn't necessarily quality, of course. As Kuff points out, two of the 34 are Republican stalking horses: the omnipresent political shape-shifter Sam Tejas (HD-133) and the odious-beyond-belief Tea Party Democrat Lloyd Oliver (HD-134). If Allison Sawyer has her ducks in some kind of marching formation, she should get sufficient turnout to rescue her district's Democrats from grave embarrassment and take on vulnerable incumbent Sarah Davis.
The only districts left unchallenged are Republican strongholds: Dan Huberty's HD-127 (Greater Kingwood) and Briscoe Cain's HD-128 (Baytown/Deer Park/La Porte). Green candidate Joseph McElligott challenged Huberty in 2016 and took 5.77%, just less than half the haul for Libertarian Scott Ford. McElligott will not be running as a Green in 2018.