Balance: A handful of weapons cannot me stronger than the rest in any situation, rather, each weapon needs to have its strengths and weaknesses, and perform better in different circumstances.
Uniqueness: I don't like to see a pack where I can say, "Oh look, another AK47/MP5/M16 reskin". Those packs, in my mind, are complete garbage unless you are trying to make a period-accurate TDM where each side is represented by competing manufacturers. If you are trying to make a generic TDM or FFA/DM, then it is appropriate to have generic names and generic functions.
Ease of Use: If you gotta go through 4 loving animations to reload or some stuff, in my mind, this pack is garbage. Once again, if you are going for the hyper-realism thing, then that's fine and it's not my cup of tea. Most people want guns which can be reloaded using a single butting press or alt click. Additionally, if you are displaying ammo counts, make this intuitive as well.
Variety: Everyone wants a weapon pack which is a multi-tool, else why would they download it. They want weapons which can operate effectively at multiple ranges with the same action and also has various other firing actions. You should be able to fulfil every conceivable role in a Blockland TDM or FFA/DM, however, do not add too many guns. Players are never interested in long download time, and server owners don't want idiot players arguing over loadout semantics. If you have a few select choices, then these deliberations can end quickly.
Good Media: Most know that realism, even semi-realism is not compatible with the Blockland style. Weapon models need to be blocky, using hexagonal or octagonal cross sections whenever possible and use beveled edges when tasteful. Visual effects, most importantly, particle effects, need to be tasteful and not to presumptuous. Last but not least is sound quality. Usually, this is one of those things that gets downplayed a lot, but when the creator puts extra effort into this, you end up with a lot of people praising the sounds that the pack uses.
Widespread Usage: The quality of a pack in part is due to how many users use the pack. People are not interested in learning how to use and balance a new pack which operates similarly when they have 2-3 others that they are used to and are fully proficient in using. This isn't usually within the creator's control, but following the above steps will help you achieve this one as well.