When a modpack consists of 100 mods thrown together because they happen to be popular, not because the modpack creator thinks they are actually good in modpack. (Hats)
When a not-very-good mod becomes popular because its creator is well known (Hats, Translocators)
When a modpack consists of 100 mods that don't fit together. A perfect example is when you have a GregTech-centred pack, and then add Thermal Expansion without changing any settings - or even if you do, because TE does not have enough config options to make it match with GregTech (In this case King_Lemming himself recommended removing TE).
Sometimes, when this forum doesn't have a dislike button.
When a mod adds a new feature that's a duplicate of something you already have, and doesn't let you disable it (Extra Utilities/BuildCraft; OpenBlocks/Liquid XP).
Related: When a mod has a feature that doesn't belong in that mod (Applied Energistics grinder; IC2 crops and booze; Forestry bees)
When a mod's development team doesn't listen to feedback (basically every mod but especially Forestry, IC3, GregTech).
When mods expand in ways that break existing worlds (Forestry, IC3, GregTech after 1.3)
When people enforce rules that don't actually exist (GregTech's ore dictionary stuff)
Related: When people condemn modders who don't use Forge.
Related: When people act like Forge is the best thing ever.
The fact that Forge doesn't have a system for preventing duplicate items (dusts; ores; ingots).
People who say "It's open source; if you want a change, change it yourself instead of asking me!"
People who say "If you don't like it, don't use it."
People who say "If you don't like it, just [perform time-consuming task here]!"
People who say "It's free, so it doesn't have to be good."
Edit: When mods stay in perpetual beta (IC3).
Edit: When IC3 calls itself "IC2 experimental" or "IC2 2.0" (because they don't want to admit they're changing everything?)