For example, players who either spend their starting gold on combat gear, but not items like bedrolls and rations; or who, at char creation, simply ignored the equipment packs and just loaded up on the combat gear and tools?
While I don't want to be overly difficult, I feel that an adventuring party with only one member carrying their backpack, bedroll, waterskin, and rations is... odd/lazy/lame?
I'm thinking about a small boon that's both desirable, but balanced.
What you're experiencing is a mismatch in what you all expect the actual game to be. As such, a boon will likely not make up for the confusion — at best it will be inexplicably ineffective at altering the players' choices, and at worst it will exacerbate the problem.
You see roleplay and adventure in a believable world as the essential activity of the game, and perhaps the one player who kitted their character out with gear reasonable to an explorer is also on your wavelength.
The players who are buying only combat gear do not see that as the essential activity. To them, the game that lies ahead of them in their imaginations is one largely composed of combat and mechanical puzzles — mechanical in the sense of game mechanics, not levers and pulleys in the game world. Effectively building their character for the (expected) challenges of the combat-focused game that they are anticipating is of utmost importance, else they will fail, and the failure will be entirely their fault for making a "bad" character. They're naturally avoiding that at all costs, which includes wasting money on stuff that they won't need — or if they will need it, will appear conveniently within the adventure nearby where it needs to be used.
You can perhaps see why these two sets of beliefs about the future activity of the game could both arise from "let's play D&D," and why they are going to have many points of friction with each other. You are likely just seeing the very tiniest tip of the iceberg here, in terms of the ways these players will conflict with each others' and your own ideas of what the fundamental point of play is. To use someone else's analogy, it's as if you've sat down and said "let's play cards," and you have Go Fish in mind while others are expecting five-card stud poker. Without discussing beforehand what you're actually doing and what the point of it all is, you can easily get partway into a game before it starts to break down under the tensions of people trying to play the game "right," according to how they expect it to work.
A minor boon seems like a good idea — positive reinforcement, minor enough not to make lacking it effectively a punishment, and an encouragement for the players to alter how they play to get the little bit of extra benefit.
Except it's not going to work that way.
The fundamental problem is that you're all trying to play a different game, and a small benefit that makes sense within the game you're trying to play is not going to work as expected in the game they're trying to play. The root of the problem is deeper and not going to be fixed by addressing a minor symptom.
Are you playing a game where emotive performance is paramount? Where clever problem-solving using the tools at your disposal is key? Where skilled leveraging of the interactions of combat rules leads to success?
These are all different goals. They're not incompatible, but not all caring about the same goal is incompatible.
To fix this, you have to sit down and have a conversation about what "playing D&D" means to each of you, and find out if you have any common ground. Yeah, this may seem like taking a game too seriously. The alternative is to just forge ahead without worrying about it — maybe things will work out organically somehow, maybe they won't, and taking that gamble is totally an option.
If you want to take a more measured approach, or if the game group ending messily is something you want to avoid, you have to get on the same page before you continue much farther down this road and the players get more invested in their personal views of how the game "should" be.
Handily, there is The Same Page Tool for this purpose. Basically it's a checklist of things that are currently all assumed by you and your players, but which are mutually exclusive ways of and purposes for playing a roleplaying game in the first place. Using the tool to explain what you're assuming (it's not a survey for everyone to fill out individually, you shouldn't have more than one copy on the table), gives you a starting point for explaining things about the game you intend to run for them, things that probably you don't even realise needs explaining or even knew existed as points to differ on.
Fill out your personal vision of the Same Page Tool, and bring it to your group. See if they agree, where they disagree, and have a conversation about that. Maybe you can all adjust and compromise — it's likely that you can. Maybe not, and you'll have to figure out what that means for you as a group, then. Regardless, knowledge is power. It's way better than groping around in the dark and having unhappy play experiences, because roleplaying is supposed to be enjoyed!
Ask yourself the following questions:
If you can answer 'No' to all of these things, they just have what ever gear is needed, and you can move on.
Why don't you just ask them what they are sleeping on when night time rolls around. There shouldn't be much argument (tho there will be), when you give everyone without camp gear a -1 to attack rolls, or defenses if you choose, the next day for being stiff and sore from sleeping on the ground after a hungry night of not eating.
There are two types of players generally when it comes to gaming:
Hack and slashers - just go around engaging in combat as much as possible and foregoing all the rest of the "in-between" stuff.
Role-players - people who engage in the "whole" experience. They enjoy travelling, dungeon crawling, rp'ing - the whole adventure, not just the combat.
You either need to push the party in the direction you, as the game master, wish your world to be. OR tailor your adventure to the party's leanings. If they all agree they want to be hack and slashers, then don't worry about provisions and combat forth.
Quite honestly, just ask them how they want to play....or tell them how your adventure is run. Talk about it, instead of trying to end around with a reward system.
As an aside, i'm not too keen on KRyan's answer above, but everyone is entitled to their opinion. Having played both styles of game, i find the role-playing style to be a much more involving, enjoyable system, but then again i play with an older crowd. Younger players tend to play more hack and slash style. I think D&D as a game system has plenty of rules to accommodate any style of play...
Instead of simply giving a uniform 'minor boon' to those who come prepared for life in the rugged wilds instead of expecting to be provided for, work provisions into the fabric of play. Food and water, especially, can become part of the adventure, as the party balances their wish to travel lightly -- thus living off the land and the fruits of their hunts -- with the desirability of the stable, known-good food source prepackaged rations provide. This may require sprinkling in lower-level bestial encounters among what the adventure usually provides, though: dire boars are much better eating than troglodytes or elementals!
If the people not buying gear did so to get a money advantage, then punish them, don't reward players who do follow the rules.
If they ignored rations, have them weaken and eventually starve to death.
If they ignore a tent, animals could steal their food, an ambush using bows is going to target the guy out in the open instead of the guy hidden in a tent.
But, as KRyan says, most people don't buy those things simply because it is not fun to worry about them. Whenever I DM, I just allow the players to pay a reasonable amount to write down 'adventuring stuff' on their equipment list and forget the whole thing.
