Gothic architecture
It was principally the widespread introduction of a single feature, the pointed arch, which was to bring about the change that separates Gothic from Romanesque. This technological change permitted both structural and stylistic change which broke the tradition of massive masonry and solid walls penetrated by small openings, replacing it with a style where light appears to triumph over substance. With its use came the development of many other architectural devices, previously put to the test in scattered buildings and then called into service to meet the structural, aesthetic and ideological needs of the new style. These include the flying buttresses, pinnacles and traceried windows which typify Gothic ecclesiastical architecture.
Gothic architecture
The pointed arch, one of the defining attributes of Gothic, appears in Late Roman Byzantine architecture and the Sasanian architecture of Iran during late antiquity, although the form had been used earlier, as in the possibly 1st century AD Temple of Bel, Dura Europos in Roman Mesopotamia. In the Roman context it occurred in church buildings in Syria and occasional secular structures, like the Karamagara Bridge in modern Turkey. In Sassanid architecture parabolic and pointed arches were employed in both palace and sacred construction. A very slightly pointed arch built in 549 exists in the apse of the Basilica of Sant'Apollinare in Classe in Ravenna, and slightly more pointed example from a church, built 564 at Qasr Ibn Wardan in Roman Syria. Pointed arches' development may have been influenced by the elliptical and parabolic arches frequently employed in Sasanian buildings using pitched brick vaulting, which obviated any need for wooden centring and which had for millennia been used in Mesopotamia and Syria. The oldest pointed arches in Islamic architecture are in the Dome of the Rock, completed in 691/2, while some others appear in the Great Mosque of Damascus, begun in 705. The Umayyads were responsible for the oldest significantly pointed arches in medieval western Europe, employing them alongside horseshoe arches in the Great Mosque of Cordoba, built from 785 and repeatedly extended. The Abbasid palace at al-Ukhaidir employed pointed arches in 778 as a dominant theme both structural and decorative throughout the façades and vaults of the complex, while the tomb of al-Muntasir, built 862, employed a dome with a pointed arch profile. Abbasid Samarra had many pointed arches, notably its surviving Bab al-ʿAmma (monumental triple gateway). By the 9th century the pointed arch was used in Egypt and North Africa: in the Nilometer at Fustat in 861, the 876 Mosque of Ibn Tulun in Cairo, and the 870s Great Mosque of Kairouan. Through the 8th and 9th centuries, the pointed arch was employed as standard in secular buildings in architecture throughout the Islamic world. The 10th century Aljafería at Zaragoza displays numerous forms of arch, including many pointed arches decorated and elaborated to a level of design sophistication not seen in Gothic architecture for a further two centuries.

Question: What was the most important single design aspect that separated the Gothic style from the Romanesque? 
Answer: the pointed arch