My approach to modded Minecraft is actually playing Vanilla with the most content of the mods I can. I play with magic mods right? Mainly because I don't really like heavy automation and I suck at redstone.
So, let's take Thaumcraft. I want to research everything in the Thaumonomicon and possibly use all the things, golems, arcane bore, wands... Ars Magica; find/craft as many of the spells as I can, reach the highest level of magic I can and find the Archmage Tower to unlock Augmented spells. Soul Shards; create exp farms along with Thaumcraft and Ars Magica mechanics. Forestry; make Thaumic Bees. Magical Crops; farm stuff without having to mine for them (already made diamond plants). Twilight Forest; well, a whole new dimension to explore and mobs to kill. Xeno's Reliquary; use the magical stuff to be a god really.
But my main goal is to kill the Enderdragon, Wither and other Twilight Forest bosses using all these magical mods. That's pretty much my playstyle and that's what I've set as goal.