I know it all depends, but am interested in what length stems people are using with threadless headsets. I've got a 110cm. I think a 100 might be better, but don't really know. Can you really notice 10cm?

I'm assuming you mean mm, not cm.

I've got a 120mm stem... 10mm can make a difference to comfort levels.

Sorry, I did mean mm. Also, who makes a plain, simple non MTB looking stem?

when your hands are on the top of the drops (if you have them) & the bar covers the front hub, you're good.

This question is way too vague for any answer to be of any help.

All this advice is somewhat meaningless (beyond establishing a ballpark, whish 3MTA3 did) because it depends on what you want to do and especially on the length of your tt and where your seat is. "If it feels good it is good" is the only real rule, but for some purposes (racing) even something that feels sort of bad can be good.

How big is your bike? How big are you? What kind of bars are you using? As jim-bob says, we need more info. Can you notice 10mm? Sometimes it makes a huge difference.

My advice is to buy a stem from a shop. If they are any good, they will let you bolt a few lengths (and stem angles) to find what works best for you.