Dimensional Doors does a lot of things pretty well -- pocket dimensions to set up a hidden spot, doorways linking far off areas, small extra-dimensional dungeons to explore, a useful if tricky-to-use weapon. It's incomplete, so limbo and dungeon dimensions aren't really as dangerous as they should be, but it's quite a fun mod. Personal dimensions are much better than trying to set up players with their own Mystcraft age just for a small house worth, even if it does have limits that Mystcraft doesn't (no beds, no sun, no minerals, 3x3 chunk cube).
Millenaire is one of those mods that shouldn't be part of a mod pack (and the authors won't give permission even if you wanted it anyway), but it's a very interesting variant that drastically tips the way you play Minecraft without the large systems-scale focuses of things like Thaumcraft or IC2. Very big mod, makes things a lot less empty and a lot more alive. Somewhat tricky to add to a modpack, due to dumb ID conflicts (Buildcraft?).
Ars Magica is... complicated. I really, really don't recommend any of the versions for Minecraft 1.4.7, since they alternate between buggy and feature-broken, but the current release for 1.5.2 is very robust. And it adds a lot of great things, starting with a good magic system and working up to mini-dungeons, a power distribution system, exploration rewards, so on and so forth. A lot of beautiful art and good worldgen design when it works right. It's disruptive, though, with wizard's towers and dungeons and creature spawns everywhere that are all hard to beat without using the Ars Magica systems. Like Millenaire, it also has ID conflicts that it /really shouldn't/, complicating install (what is it with these folk and Buildcraft?), and tends to be finicky about being added to a modpack. The magic fabricator also isn't that powerful in most situations -- especially contrasted to mob grinders or EE3 --- but it can screw up balance with other mods.
Chococraft's got some dependencies on DrZhark's custom creature spawner stuff, and last I heard he's not very quick to give permission for modpack use.
