Ingame documentation is often considered vital, mostly due to the outdatedness and/or inconvenience of wiki's (but keep updating, it's good to have :D). Personally I consider books just as clunky as wiki's, and have always liked IC2's tooltips to see power use, for example. However, it would be even better if everything (well, everything that needs it :P) gets a tooltip with some info.
For starters, batteries and storage units should list their power tier, instead of only the items doing this. I think the power tiers should be changed to voltage tiers (eg. Tier 3= HV) but if you have a reason not to do this it's fine. Protection value of the armours would be nice, but not a nessecity. Lots of people are often confused on how to obtain Reinforced Stone now, so a tooltip for the Iron Scaffold saying "Spray with CFoam to make Reinforced Stone" would clear up a lot. Some of the machines(like the Block Cutting Machine) are lacking power usage/input information and need fixing.
And probably the biggest of all would be reactor component information. For example a Uranium Fuel rod will say "Generates 5EU/t and 4 heat per tick". This could also adapt(similar to overclockers) and change the tooltip depending on the surrounding components in the reactor. I don't know how hard that second bit will be to implement, but I'm sure you guys could pull something off