I like the Technic Pack Minecraft mods, so I've not been very happy about the fact that they're several versions behind. It's a little bit about new vanilla features I like, but mostly it's about the little functionality tweaks that, now that they exist, are hard to play without. Like, it's no fun to lose the ability to shift-click armor on and off. But a new pack has come out -- the Feed the Beast pack, which comes with a load of awesome mods -- like ExtraBiomes XL, Buildcraft, Industrial Craft, and two dozen more. It's up to date with 1.4.2, which is 0.3 updates behind, but it's still a lot better than 1.2.5.
I'm very busy today, so I was only able to spend about ten minutes playing it, having told myself it was justified because I was going to blog about it. So, I didn't get to do much that wasn't familiar to Vanilla -- mining new kinds of ores isn't hugely more fun than mining the vanilla ores, and the biome I spawned in (Green swampland) wasn't a huge departure from the normal biomes.
But I am super looking forward to when I have more free time and I can spend a decent amount of my free time building a massive lab. I'm thinking my first long-term goal is to build an array of nuclear reactors powerful enough to make enough uu-mater to craft a diamond a minute. (That's one thing I'm a little disappointed about -- it looks like Equivalent Exchange hasn't been updated to the new edition, so no insanely powerful magical stuff-generators.)