lollll 11 gigs in the nether nice. I'll have to look up what horizons is.
http://wiki.feed-the-beast.com/Horizons
Key points that I have discovered so far:
-Tinkerers mod and metallurgy mod.
Tinkerers allows you to build your tools from a variety of different materials with a variety of different stats, for example: A flint pickaxe with a wooden rod and paper binding. Or a sword made out of cactus. Also allows to repair stuff.
It also allows to build advanced tools, like the hammer that works like a pickaxe, it just mines 9 blocks at a time.
Additionally, you have smelteries for metals. They are a little bit difficult to set up, but once they are running you can make alloys and stuff.
And this is where metallurgy comes in. It dramatically increases the amount of different alloys you can make. Totally dig it.
-Biomes ´o plenty and alternative terrain generation
Gives more diversity in biomes and also a more "sensable" generation. Stuff like higher places being more dry and cold and places near the ocean being very green
-Tropicraft
For some reason I really like this mod. It introduces a new kind of water, that works just like regular water but is a reskin. But it is a very nice reskin. Also gives you some "beach"-themed recipes with bamboo.
-Agriculture
Oh boy. Introduces 20-30 new different kinds of fruit and vegetables and recepies that go with it.
And that´s just a handful of mods. Right now I´m working on Thaumcraft, which is a magic type mod, where you have to gather essences and can craft magic spells. Very complicated, so I´m still looking into it.
For tech guys there is rotary craft, which is all about centrifuges and Thermal expansion, which is a energy generation mod.
In total its more than 100 mods. As I said, I would wait until a 1.8 modpack comes. It is really far more stable that way.