I try to keep as many features free and unlimited as possible. Some things have to be somewhat limited to paid members unfortunately.
In the earlier days, when the paid tier was called Gold (and Supergold) membership, comments were a paid feature, but when I updated the site to v2, I decided that comments are just such a core feature that it's not cool to paywall them.
Image hosting also used to be a paid feature, but is open to everyone now, but free tier members have less storage space, out of necessity (but there's also an option that will shrink your images to be more efficient, so you can store more of them).
My main principle, which I have discussed many times with people I know, is that I want to allow people to access as many features as possible, with as few limits as possible, and I think that will lead to people subscribing to the Support Plan because they feel positively towards Diaryland, and it has meaning to them.
This is probably also the place to say that there is no place for ads on Diaryland (I guess this goes along with the Interface section above). It would be such a bummer to have them, but also I think that long-term it'd be a mistake anyway. If you ever see ads on Diaryland, something went very wrong.