Why doesn't the iMac have an HDMI (High-Definition Multimedia Interface) connector? Thunderbolt and USB ports are nice. However, HDMI is getting smaller even as it gets bigger (more on that in a moment).
But let's back up. HDMI is an interface for audiovisual equipment such as high-definition television and home theater systems. It’s able to carry a bandwidth of 5Gbps (gigabits per second), which is purportedly more than twice the bandwidth needed to transmit multi-channel audio and video. HDMI provides an uncompressed, all-digital signal.
HDMI makes perfect sense for inclusion in the iMac. After all, it's a computer that's oftenintegrated into home entertainment systems and/or used for gaming and movie viewing.
Apple developed a Mini DisplayPort in the past. Perhaps it's time for a mini-HDMI connector. Until that, or a full sized HDMI cable appears, you can see how to connect a HDMI-less Mac to HDMI devices here.