Personally for me a mod pack isn't fun if it is absurdly difficult, I hate mods that make it harder for me to get the resources I need. I'm also not a fan of Story-Driven or questing packs as I don't want to be told HOW I should be playing a pack, I wan't to make my own adventure. But that's just my opinion on what's fun.
As for preferred mods everyone has their own style they like. Personally I am in LOVE with the industrial based mods like Immersive engineering, Pneumaticraft, Railcraft, and Mekanism. As I like to make huge mass production factories. But some people go for more of a future tech, some go for a magic theme. some go for those crazy ramp up the difficulty to the max packs.
You can't make a mod pack for everyone because no computer could run it.
When making a mod pack it's more you have to ask yourself what YOU want to play and what people LIKE YOU would want to play. Make a modpack for a target audience, rather than crowd storming up one. You'll come up with a much more coherent pack rather than some some all in one reject.
Also this is a problem I see with a LOT of mod packs, even if your computer can handle it, try to keep the RAM and CPU ghz usage under the general average for home computers. Don't add in a bunch of mods to make it beautiful just to have it be unplayable be the average Joe.
Now there are some staples that almost EVERY mod pack should have:
-Extra Untilities- Nearly every pack has this, and for good reason, it's packed full of useful things that make the mundane tasks a bit less tedious, and allows the player to enjoy the rest of the pack. Not to mention it adds a few really nice features all by itself
-Some form of Storage System- 2 of the really popular ones are Applied Energistics 2 and LogisticalPipes. AE is more futuristic and shiny. Logistical pipes has more of an Industrial theme. There are many other storage system, but these 2 are the most popular.
-NEI and OreDict- If you don't use these you might as well not even make a mod pack, nuff said.
-Decorative Block Mods- Things like Chisle help breath variety and life into the builds in a modack, you can have all the awesome mods you want but will they really look good in a vanilla cobblestone shack?