You know, I was pondering back in 1.4.7 about the viability of removing Buildcraft entirely from my mod pack, however it seemed at the time as though there were several things found in BC which people found absolutely necessary... quarries and gates seemed to be the two biggest things.
Honestly, though... if MFR updates to 1.7 with TE3, I might well dump MJ entirely, assuming there can be some compatibility between RF and UE.
With TE, I have a lot of the 'core' functionalities... ore processing, utility, transportation of anything I want moved around, plenty of built in automation. With Mekanism, I get the Universal Cable and the Energy Cubes which, assuming there is compatibility between RF and UE, can be used to convert to power anything else. It also has several passive energy generation systems in its own right, and a different method of ore processing for those who want something a bit more complex but still able to be automated. With MFR, I have automation of growing things and, eventually, automation of ores. However, it is not -easy- to automate the ores, which I feel is a very good thing. I also feel that the power required to automate the Mining Laser is appropriately balanced. With AE, you solve your storage problems.
Toss in TiCo and Natura of course, because both of those make the world interesting again.
It's an exceedingly tight system, one which provides nearly everything the new player would want or need in a game. It would introduce several concepts to the new player, and would be resource-light so that you could continue playing it on 'legacy hardware'. In effect, if you could run vanilla MC, you could run ShneekeyCraft.
Forestry was fun... it really isn't anymore. Not to me, anyways. Sure, with ExtraBees, it is effectively 'all the renewable things', but... it's too steep a time investment for me. Bees are a boring grind, not entertaining in the least. Yes, they have enormous return on investment, but the down payment is sufficient that I do not wish to mortgage my free time to pay it. About the -only- thing I'd want Forestry for is the backpacks.
Railcraft was fun... and to some degree it still is, I suppose. Then again, I started out with model trains back in the day, so I happen to like all the rails. However, with the nerf-bat swung at boilers, they've gotten to the point where either you are cheesy in your fuel generation or it simply isn't worth the effort.
So with TE, MFR, AE, TiCo, and Natura as 'core' mods, I can build a pack which is still enjoyable, but won't require an excessive amount of resources.