You've pretty much hit the nail on the head with the collection of mods comment. Some add major new features and tons of content, others are add-ons to the bigger mods adding a couple of new items that fit in another mod's theme. My understanding is that it started off as a collection of mods that the curators managed to make work together.

In its current state, it's a launcher for clients, essentially a package manager for minecraft mods, with several public packages available (Mindcrack, ftb) and also the ability to add private packages, or make your own by editing the existing ones.

What makes the mods special I guess is a combination of the sheer amount of content they add (energy systems, different realms with different monsters and bosses, more armour, ores, different biomes, beekeeping aka pokemon) and the way they all work together either by design or by glue mods.

Rather than reading up what all 30 mods do, I'd look at the following mods and see if any interest you. Please note, a lot of these mods add new types of resources to the new game, e.g., copper, silver, tin, lead, uranium, rubies.

Buildcraft + forrestry + thermal expansion: Buildcraft is one of what I consider the base mods, which adds the idea of energy called Minecraft Joules (MJ) which is used to power their machine blocks. Also adds pipes to transport items, liquids and energy. You can use these mods to do things like automatic tree farms, beekeeping, quarries (autmoatically mines out an area) lava pump.

Railcraft: Adds a couple of things that interact well with buildcraft, steam boilers, more buildcraft engines (engines generate MJ), liquid tanks (for storage) and tons of different rail stuff than in vanilla minecraft, e.g., launcher tracks, high speed rails, locomotives.

IndustrialCraft 2 and optionally Gregtech: Adds its own energy system, EU, which is used to power its own machine blocks. These machine blocks are all about item processing, e.g., getting ores and turning it into 2x or 3x as many ingots as you would have normally received in vanilla minecraft. Gregtech sort of extends this processing technology tree, adds some new items but also 'balances' all of the mods in the pack, editing their configurations so that they don't enable, in Greg's mind, an unfair amount of resources to be generated for a given period of time. Whether or not you agree with such a viewpoint is one of the distinguishing factors between the modpacks.

ComputerCraft: LUA programmable computers and 'turtles' (think logo) that can interact with the environment. MiscPeripherals is another mod that adds more methods for computers and turtles to interact with the minecraft environment and other mods.

Mystcraft: Adds the ability to generate new worlds, with funky settings like, multiple suns, always-night, dense ores, only plains biomes, purple sunset, and many more. Lets you teleport between realms.

Twilight Forest: A new realm, like the end or the nether, which is a more fantasy/magically oriented realm than normal minecraft, with entirely new creatures, monsters and bosses. I believe the idea behind it is if you're looking for a more magical setting than normal minecraft you could play entirely in this realm.

Extrabees: Really a small mod but has so much playtime involved. Forrestry adds beekeeping to the game, and extra bees adds a hundred or so more bees to the original 10-20 forrestry bees, with bees further down the evolutionary chain producing more advanced materials, e.g., diamonds. It also adds some machines to aid in the beekeeping process.

There are more mods worth talking about, but I should get back to work :) I'd watch some lets play videos on youtube to get more of a feel for what's going on. Just jumping into the game can be kind of overwhelming.