The DDC interface, which is an I2C bus that allows for bidirectional communications to read EDID data, so you know what you are plugging into. Hot plug detection via the HPD line, so you can tell if you are plugged in or not. Embedded 5V power, very useful for conversion dongles, ethernet, probably more things I forget. HDMI has a lot of pins, which makes DIY cables difficult, however they are useful.

You (your hardware) can tell if SDI is plugged in if the line is terminated with 75 ohms (SDI allows looping through to multiple drops)

I can't think of any time I've ever used anything on a HDMI that uses power -- my HDMI to SDI converters (or vice versa) have separate power so it can't be very reliable and/or powerful

I guess EDID data can be useful in the computing space for feeding back from the monitor.

You’re not wrong, but the biggest thing is wider support. I’ve only seen SDI available on expensive camera gear (but I don’t look that hard), but HDMI is ubiquitous. It’s sad DisplayPort didn’t end up winning instead, but I guess TV manufacturers are members, so they’d put it in the TVs. DisplayPort still has a stupid “membership fee” but at least it’s royalty free.

SDI doesn’t have the built in encryption so you’ll never see it on something that supports playback.

DRM really does ruin everything, eh?