In the 4.0 nightly community builds, extra transformation configuration was added (code from 4.0.1 and 3.4.8 service packs) to disable the creation of thumbnails in general and disable thumbnail creation for a given mimetype based on the size of the file. This mechanism is based on a customisation that has been applied on a number of customer sites. Unless you have no need for thumbnails at all or already have this customisation, it is probably better to use the approach describe in Content Transformation Limits.
Turning off all Thumbnail Generation
All thumbnail generation may be turned off by setting an Alfresco Global Property.
Turning off selected mimetype Thumbnail Generation
The thumbnail-service-context.xml file contains spring bean configuration that control the generation of thumbnails.
The 'create-thumbnail' and 'update-thumbnail' beans have a 'mimetypeMaxSourceSizeKBytes' property that may set maximum size limits (KB) for each source mimetype. If the value is less than zero, a thumbnail will be generated regardless of size. A value of 0 turns off thumbnail generation for a given mimetype. The following is the original configuration. Note the use of Alfresco global properties for values so that it is straight forward to change the configuration (all values have been defaulted to -1).