This was the first time in the series a "mindless" Titan was shown to have the capacity for speech and complex emotion (reverence).
The episode follows two parallel timelines: the present search by the Scout Regiment and the past account written in a diary. Shingeki no Kyojin OVA - Episode 1
The episode introduced the name "Ymir" long before its significance was fully explained in the main series, hinting at a deeper history behind the Titans. This was the first time in the series