Thermal Expansion will likely give you the most familiar experience (outside going with IC2), the major difference being that the machines are much more configurable, and run on BC power, rather than IC2.
That's not the only start possible in the pack, though. Tinker's Construct brings the smeltery for the uber-efficient. This can be built from basically raw materials (sand, clay, gravel) and gives you quite a neat way to double your ores off the bat, aswell as offering up the metal materials for the TC tools (easily the best for early-mid game).
Alternatively, there's factorisation, which, while it has an expensive initial cost (9 diamonds and either a ton of silver, or a Railcraft steam boiler), it does give you the opportunity to treble your ores.
There's also Dartcraft, which can place the ore doubling on the tools, meaning no processing required.
Other points to note:
- Forestry farms are radically different. They are now multibocks which allow you to do multiple resources, planting and harvesting without a massive space investment. If you prefer the old route, Minefactory Reloaded is probably closest to it.
- Storage and automation solutions. RedPower hasn't been updated to 1.5.2, so there's no sorting machine. There are, however a multitude of other potions, from standard buildcraft, logistics pipes, Applied energistics, or even pipes and transfer nodes. Details can be found on the wiki, but there are a number of options, each with the plusses and minuses.
- Power. I'd say that the biggest thing to happen to Modded Minecraft since then is that it finally got modern and made itself steam power. This is an easy way of getting high efficiency always-on systems.
I think those are the only major things I should point out, but other will likely correct me.
