It seems most people don't like to trade the mini Dow. Is there any reason for this? Or any sort of general knowledge on why other instruments like the ES are so popular?

I don't know, I'm just starting with futures and YM looks a little more user friendly with lower value ticks and $5 multiples.
Even if the tick value is too low for somebody, all you need to do is increase the contract size.

Is the YM seen as a beginner's item? Or just used for training before moving to the big leagues?
I'm not insinuating anything, just curious about the general opinion on the mini Dow.

I equally trade the YM, NQ and ES, no bias at all from my side. More often than not, the patterns are similar across these indexes, but not always. So I just pick the one that has the highest probability of success and trade it.

Believe it or not, you might have better fills trading the YM intraday especially if younare into scalping andd if you place your orders last minute. The ES is so thick that you will not be filled unless price crosses your level of course by at least one tick

Ok, so I was thinking the ES would be preferred due to higher volume, and it may be true.
But in some cases that may work against somebody fighting for that extra tick.

I guess it makes sense, more people trying to get that X price means longer waiting lines.