Most of the modpacks I've seen fit one specific 'genre', you build up along a progression tree, spending a lot of time grinding in each step, until you've achieved perfect automation. And while I can see the appeal, it's just not for me. Pretty much anything that's simply "climb the progression tree," doesn't appeal to me.
I want something more unique, that tries to play with Minecraft's features in more unusual and fun ways. One thing I can think of is Journey To The Core, I never played too much of it, simply because it was a bit too outdated by the time I found it, but the general concept of trying to reach deeper "levels" in such a hostile environment was enticing to me.
Also, as I'm not extremely experienced in the flow of most mods, packs with quest books would be preferable.