Since the topic sometime came up on chat, why not dedicate a thread the what Coby called "The Loop"?
The loop is what keeps players jump back into the game after a losse/win, encouraging you to play another round right after instead closing the game and doing something else.
This heavily overlaps with replayabilty, so let's take that into account as well.
If players want to quickly play a round, let them. No grand unskippable entrance cinematcs, because after the 10th time it's just annoying. If players want to speed up things let them.
For the same reasons as above, hold back on giving any direct advice. "Hey this wall looks cracked" might sound ok the first few times you hear it, but then it just becomes annoying. A intricated detail on the wall is enough. and if you ever want to hint that for people, you can always build rooms that can introduce some mechanics (BoI for example has rooms with explosives that can accidentally reveal a hidden room for example), or have a random NPC hint it. Just don't shove it down players' throats. Since ddrop apparently gets a tutorial section (guess similar to spelunky's optional one), theres no reason to keep bringing up the basics anyway.
To make sure thing work, it gets tested times and times over again to check if after the xth cycle it still works. Test your parts of the game like this. If it works only once or twice, there's no poin in adding it. Remember, whatever you create, players will see it again, again and again, so better make it repetition proof. A 8 clue Simon Says get old very fast for example.
- Streamline the mandatory parts
This issue already came up long ago in the form of backtracking. I like backtracking, but I've come accept that it's not needed if a player doesn't care about details. But Don't reward them for being lazy.
- Reward players going the extra mile
If a player wants to spend some extra effort, reward them for that. If players want to backtrack and search every nook of a level, give them a little extra.
- Don't turn the game into an elaborted coinflip
The rng should NEVER EVER decide whether or not one looses or wins the game. And no, that will never pass as a good design if players are forced to take the bitter pill. Binding of Isaac suffers from this very much, to a point where you can almost guarantee how a run will end after the 2-3 first levels. In the same vein, don't hand out sure-wins or incredibly good items that are balanced by rarity alone.
- Allow a certain level of power to carryover
You mentioned armor that that would break after some time. That's a great example for certain rewards that can carry over is the player does well enough.
- Rewards should present more options then straight powerups. Beware of snowballing
If a player does well and gets a reward, don't snowball into a mess of "the reward from a previous challenge making a current easier which in turn will make a future challenge easier". Opt to give players rewards that fit the general powerlevel instead.
- No sudden radical genre shifts
Binding of Isaac's final levels get hit by this hard, when the game changes from a zelda inspired game into a combination of bullet hell and elborated coinflip. Previously learned enemy behaviors or dodging skills are thrown out of the window, because 60% of the attacks will be just plain random. If you ask BoI players about the worst parts as lot will mention the fights against Isaac or ? ? ?, because they are completely out of touch with the rest of the game
If it's no really needed in the game or worse , if it gets ignored players or hinders them, cut it out. Does delver's drop need a cooking minigame for example? I doubt it. If you have story rooms etc, make sure to make them worthwhile even for a player that already knows all of the story, so they still get rewarded by exploring, like a newcomer would!
- One tactic or item to dominate them all. NOT
I once heard the term "reversed turing-test" and I think i fits very well. If you could make a program for you game that can play it flawlessly, there's something wrong with it. To make it simpler, "don't make anything THE BEST". If I could use a Tier 9 Hammer or a Tier 9 Warhammer, both need to be an equally viable choice long term. Everything has strenghts in different situations, but nothing is best at everything. DDrop does dodge that problem a lot thanks to weapon proficiencies already. Same goes for classes.
- Similar/Familiar but not the same
Players should remember parts of zones and their trademark stuff. The dungeon might be random, but it's theme should not be. Not a real problem for DDrop I think though.
Here are two more personal bits:
- Don't make unlocks stronger than standard, make them different!
BoI has the problem that 3 of the playable characters are outright useless because they are based around a gimmick (Eve, Maggy and Smason). You spend your whole run with trying to make these characters do what other characters can do from the get-go. To contrast this BoI has D6-Isaac and Cain, both of which are characters that on top of being decent also have the ability to influence the game's RNG itself, making them superior to any other choice. Same for skills. There's no point if ever having a skilltree, or well, even attempting to go deeper instead of exp farming, if there is stuff like pure +10% attack speed, because it's obviously the best choice.
- Classes are meant to present alternative playstyles, not alternative skins with slight alterations
See the above about BoI on how it's not done. Check NT's characters for a textbook example of how to do it. Each character brings something different to the table, not just so boring offspring of the base character. And keep specialized characters specialized. Let's look at NT's character called Meltling: The guy only has 2 hp, meaning that unlike every other character, he's a one hit wonder, but in return he gains more exp and thus gets bonuses more fast. People that don't want to play such class are free to skip it. It's the same dilema with people hating the sorcerer or gladiator. You get nothing from "standardizing" classes, because all it does is robbing people that want to play a different style the chance to do so, while people that hated said class will only get 2 slightly different classes. A loose-loose.
- Later level combat increase complexity instead of widening a powergap between players and monsters
This should play along with tiered weapons very nicely in your case.
I'd love to hear other people's opionons on what keeps them "in the loop"!