Until Liquiducts, I'd have said Valve Pipes was the mod I miss the most, particularly when piping steam around.
I do still miss the insertion pipe from ABO. That thing was dead useful for automating bee products.
The only mod I'm patiently waiting for right now is Modern Warfare, which is pretty much a Forge port of SDK's Guns mod, minus a few things that didn't need to be included. I'm trying to find a decent gun mod that doesn't suck that is Forge compatible for the current version. The only one that comes close is Ferullo's, and that one has WAY too many guns. Maybe SimpleGuns will do the job, I dunno. I just find it odd that I can make mining lasers, quantum suits, and anti-gravity... but I can't make a gun. There tends to be a dearth of ranged weapons included in mods, and this is a gap I hope to fill with one of these.