App sizes differ greatly from just 20MB for news or photo apps that pulls content in from the web, to 200 megabyte for puzzle games and even 800MB for heavier titles.
Capacity is a balancing thing because in reality no-capacity is enough as anyone can fill an iPad if they really wanted to. I have friends with pages and pages of apps in folders but how many of them do they use every day? Possibly only 3-4, most of the time spent on the iPad is in the browser.
But I do have to say that offline data can eat quite a bit of capacity. I had 6 email accounts on my iPad and enjoyed having 2000 stores on the device for quick retrieval and search, surprisingly the email data was in the region of 1-2GB and so was browsing history.
If you're smart about it you could use spotify on the iPad and avoid having songs eating up most of your capacity. I used to own an iPod Touch with 64GB and I managed to fill it before it got stolen, so when I bought a 16GB iPad 2 I was scared that I would struggle but surprisingly I adapted well to the lower capacity and was even wiser too, and stopped impulsively download apps that I would use once and then just keep on the device.
I become more efficient with my capacity.