It's more the new mods than the new features of existing mods for me.
Yes. This is the reason..
My computer could never play the monster pack. It's just too many darn mods. With the slightly more focused packs, I have no issues running them. I personally have moved off of 1.6.4, and am running a custom 1.7.10 pack. I built this primarily due to the new Thaumcraft, but have expanded it to include a variety of magic based mods (Blood Magic, Witchery, Botania) along with forestry, railcraft and buildcraft (they are kinda tech-y, but not overly so.) I find with TE and AE available to me, I build the same exact setups because they are reliable and so easy to create. With these mods missing from my current pack, I'm having to find all new ways of playing the game and getting the resources I need. This has caused me to need to think a way around the issues I have. I would never have done this with a include-all-things pack simply because there is no reason to have to. Also with a focused pack, the pack maintainer can balance the play through much more thoroughly. It's hard balancing a pack like Monster. There are so many ways to do something and along with the cross-mod interaction, a few ways to exploit (or significantly outperform) other building options, that even with all the options, people still bring it down to just a few types of builds.
I never played with BM until AgS, and I have never played with Botania until my custom pack. Magic World 2 was my first exposure to Witchery, and Magic Farm 2 was Pam's Harvestcraft and Hunger Overhaul. All of these packs I've played were due to it having one or two mods I was familiar with as well as several new (to me) mods that looked interesting. The next mods on my list to try out are Reika's mods (rotarycraft primarily) as well as
Professor Flaxbeard's SatanicSanta's Wondrous Steam Power Mod. I'm just waiting for someone to put together a steampunk inspired modpack showcasing it (Steam Power Mod).
The new mods are what keep me enjoying and playing Minecraft. I had stopped playing Minecraft for almost a year at one point as there really wasn't anything "new". It was simply buildcraft, railcraft, forestry, IC2, redpower and their new versions. Now, there are so many wonderful mods, I have enough to keep me entertained for quite some time.
This is kinda the reason there will never be another "show" like "Wrenched". There is no longer a "core" set of mods that everyone should know. The mods have simply gotten too diverse for this to be a possibility again. Trying to include
all the mods will simply create a pack that only the most powerful computers can play which would rule out a significant portion of the FTB community.