She's easy to harass in lane, since she doesn't get her gapcloser until 6. Just don't get close enough for her to E you, and all she can do is throw out Q's, which are also easy to dodge.
If she shoves (which Diana tends to do naturally), she's super gankable.
Diana is one of those champs that feels super oppressive to play against if you don't hard punish her early (similar to champs like Akali and Kassadin). I will say that I think she's even really good right now with the new runes, especially being able to take Resolve secondary (Second Wind does wonders for bad matchups).
I'm actually super grateful that she isn't played often because, as a support main, I hate when a champ like that is on the enemy team because I have to rely on my teammates to properly punish her, and relying on your teammates in soloQ is...unrealistic.