I've been using a Canon A620 still camera in movie mode for some time for forum posts. When creating pieces for work (I'm an engineer, not a professional videographer) I've been using HDV and downgrading, but the final results didn't always show real benefits for simple shoots. I'm typically making instructional videos, interviews... not running and gunning work.

I was making a web piece for my church and decided to compare the A620 in movie mode to an XH-A1 before filming the actual piece. What I found was pretty interesting. With a static set and good lights, the A620 did surprisingly good, even when viewing at full resolution. Once I converted down to 320 x 240, their wasn't any real advantage of the HDV.

Here's a low res link:

http://s176.photobucket.com/albums/w199/engineeringnerd/?action=view&current=080322CanonXH-A1vsA620256Kbps3x4wmv.flv


Here's a 48MB file of 8Mbps wmv for those who want to see more (get a cup of coffee, this will take about 8 minutes) :

http://www.mediafire.com/?nm4dzaehd2m

Moving forward, when I have a simple piece for the web or if I am equipping an untrained comrade, I'll reach for the A620! It's so blasted fast and easy to shoot, download, and edit.

Other thoughts:
The average point and shoot camera has a 1/2.5 inch CCD.
The average consumer video camera has smaller CCDs - 1/6 inch to 1/3 inch (HV20).

This suggests that digital still cameras will have better low light performance?
Audio capture and recording quality would be areas where camcorders are better.

Your question:
For youtube videos, I think a digicam video is definitely good enough. You don't need HDV.