Some aircraft types also do not allow pilots to use single engine taxi out procedures. In case of 737, only recently have Boeing amended guidelines to allow this type of operation and only when subjected to operator established and Boeing approved procedures.
Contrary to US, in EU-land the CTOT system prevents excessive taxi and ground delay requirements, thus the 30+ minute taxi as mentioned above could be reduced to 15- in most airports if applied properly.
As such many EU operators of types restricting single engine taxi operations or only within predefined criteria (low ATOW, defined directional turns, etc) choose not to do so.
Other aircraft types also are prone to minimum oil temperatures prior to departure, which can only be reached on for instance first flights if engines have been running for some time. On occasion (usually early morning in winter) airports are required to delay departures as an aircraft with such requirements is at the holding point but not warmed up enough for departure, very frustrating for those in the queue behind not requiring the delay.