Honestly, early buildcraft power makes most people miss ic2/rp/EVERYTHING across the board, yay the most miserable babysitting power scheme ever made with exploding machines and pipes. Mid/late game TE, redstone conduits, and energy cells make it useable power wise, but that doesn't help early on. Honestly, this version of tekkit is a bit of a let down-- but not one i can really fault the tekkit team with entirely. There is no stable version of IC2, there is no RP2, no railcraft/xycraft (tanks), the additions of AE, MFR, and TE are big assets but the game still feels light compared to the classic tekkit which was just mind numbingly insane and versatile. Tekkit to me always felt like the king of fun mods because of its versatility... and it just feel like we're being pigeonholed into doing it JUST THIS ONE WAY for every specific task. I think a lot of people are missing the versatility of tekkit classic when it comes right down to it. Hell, just think back to how many methods of mining there were in tekkit classic... people miss having a mod that lets them play their way while letting their friends play a different way without restriction and still being able to play together... I played tekkit classic longer than any other mod or even vanilla minecraft itself. It was really an amazing piece of work.
As far as ic2 goes... people miss it because it was comprehensive. It could stand alone without other mods and be complete. Buildcraft even after all this time STILL doesn't feel that way. The power feels half ass at best without RECs (conduit and cells). *shrug*
All said and done though, I think people are missing the versatility.