I don't fill out feedback forms but have some to give your modpack developers:
1. Focus on improving the overall quality of content in your modpack. By this I mean if there is a ton of useless-looking items in your modpack I can't craft with or refine into something I can use, I don't know why I'm looking at the item. Half of the time I look for something I need, along the way, I spot something I haven't seen before, and I go, what can that do for me? I end up spending half the time trying to figure out if something I've noticed is actually useful to me, because there's either a lack of in-game recipes or information on that item, and I have to stop and Google it to find out if it's worth bothering with.
2. Power usage or conversion information: A lot of the time I want to know if a particular fluid or fuel will work in a particular engine or dynamo or whatever I've got the resources for. Most of the time I can't find specific information relating to whether the fuel or fluid in question is usable in a particular engine. 1.6.4 mods used to show you exactly what engines a particular mod's fuel could be used in. Now I have to go track this down via Google, again, making me stop and waste time I could use to develop my base.
3. World-generated items that have no apparent use but clutter up my inventory anyway: For instance, there is quicksand from Biomes'o'plenty, I usually spend time digging myself out of this, therefore I always have shovels on hand to dig my way out whenever I jump over a hill that's hiding some of this on the other side. From what I understand of the real-life quicksand, it's just a lot of sand with considerable depth and saturated with water. Why can I not cook the quicksand in a furnace to make ordinary sand, and thus, make it into something usable early-game? There's tons of items from Biomes'o'plenty alone that fit into this category, they have no apparent use except to take up inventory space, and there is no recipe they can be used in. Whilst I appreciate that some of it may be eye candy, a lot of it isn't even generated as such.
4. Oredictionary and items that could be used as substitutes: I play FTB Infinity 1.7.10 a lot since the stable release came out, and I find I need substitutions for common items a lot of the time. For instance, I could make pistons with certain metals, and that's nice, but a lot of the time the balance of the substitutions seems quirky or doesn't take advantage of items you're likely to craft. For instance, copper from some mods not substituting properly across the board, or requiring a crafting method that relies on something that can't be crafted by the player. Forestry's bronze is a prime example: The only way to get Forestry's bronze should be to craft it with ingots found in worldgen, but that has been oredictionaried to yield other mod's bronze and apparently overwrote the forestry recipe without caring that bronze Forestry axes have 500 durability.
Those are some main points that spring immediately to mind, there are a lot more, but mostly I'd like to see useless item clutter reduced or made recyclable, and general improvements across most modpacks to recipe tweaks so that they're a lot more consistent and don't accidentally overwrite useful recipes.
Cheers ...
BrickVoid