Bigger VMs (180Gb, 16 cores, 8 discs up to 16TiB storage), better basic VM spec, graded discs (guaranteed SATA, archive, SAS, SSD), much cheaper basic data included, live migration, integration with their dedicated servers, managed service, generally cheaper, been around years (as bytemark.co.uk), own DC, own Internet AS + network and peering, IPv6.
Ok and they use KVM instead of Xen with virtio drivers.
Paper about their systems: http://blog.bytemark.co.uk/wp-content/uploads/2012/12/Design...