For me it is the size. Holding an iPad in one hand gets quite tiring after a while and if I want to read for 10 hours I have to keep moving to stay comfortable. When I had my Nook Tablet, the size and weight were perfect but the screen was sorely lacking and the touch input wasn't accurate enough for me.
For me the advantage of the mini would be size and cost. I carry my Kindle Keyboard with me a lot of places because it is very portable. The mini would be similar in that I would carry more because of its smaller size compared to the regular iPad.
Having tried a Nexus 7 the other day the form factor is spot on. Great for reading, quick surfing, videos and games. The 7" form factor might be more ideal for consuming content rather than creating.
Reading especially is the killer-feature on the 7" form factor, using one hand and is so much more comfortable than the bigger iPad.
I see the iPad more a general 'laptop-killer' (more most people - I use that phrase loosely) while the smaller one is aimed more at the content consumer (kindle people etc). But like all iOS devices there will be overlap.
Definitely size & weight. I want something I can carry in my purse that isn't going to feel like my whole laptop is in there. If they actually come out with the mini i'll be going back to a regular phone as opposed to the iphone and just getting a mini to throw in my purse. Internet would be cheaper on it than the plan I have on my iphone so it's kind of a no brainer for me.
Can't fit an ipad in my inside jacket pocket....but a nexus 7 does fit....I don't want to carry a backpack or carry the ipad in my hand everywhere I go....the 7 inch size is WAY more portable then the 10 inch....So Im goona wait to see if a mini is on the way...if not I will buy an N7 again...and not return in before the 2 weeks is up this time