There are a lot of disappointing weapons and armor, in my opinion. Here are a few that come to mind:
1. Phalar Aluve: this is a named sword stuck in stone that you can get by engaging in a blood ritual. It turns out to be a +1 sword, same as what the blacksmith Dammon offers (who, I might add, scoffs at his own inventory, insisting he could do better in a more stable environment).
2. Mourning Frost: this is listed as a "legendary" item. It's a staff that was broken into three parts, each held by a rival drow. What does it do? Nothing, unless you happen to be holding it while you cast a cold spell, and then it adds a single point of damage. This qualifies as legendary?
3. Adamantine Armor and Weapons: surely all the world would know of the great Adamantine Forge if it were active again, for surely such a forge would upset the balance of all the known realms... wait, what? You mean Dammon the refugee blacksmith makes better stuff? Oh, okay, hmm. (Please, please, pretty please fix this.)
4. Sorrow: sure, it's neat for story purposes, but taking a point of damage yourself every time you use it to damage an opponent? While I appreciate the story aspect and the feeling of it, who is actually using this thing? That's the point. No one wants to use a weapon like this, not without a benefit that makes the drawback worthwhile. A point of damage is a lot of damage to worry about when you're navigating combats and healing potions. I'm trying to avoid damage while dealing damage, not hurt myself. So the cool weapon that used to be owned by the paladin of Shar goes unused. (I know I can't be the only one who really wants to use this for the flavor.)
5. Ritual Axe & Loviatar's Scourge: again, I appreciate the flavor, and I can accept it with these two weapons. But I wish the benefit of using them was balanced a little better... something to tempt me into using them despite the heavy drawback.
6. Githyanki weapons: these swords that do psychic damage seem interesting at first, but they only work when the target is at less than 50% health? I'd rather have something consistent. Really, I don't want to be calculating current hit points in the middle of a fight to figure out which weapon is best to use. I like the flavor of the weapons, and I'd at least like to be able to give one of them to Lae'zel (especially since the combat to get them is so difficult, it would feel like a real prize). But there's no benefit for Lae'zel to carry one, at least at the moment. She already has better options in either the Everburn Blade or the Sword of Justice.
7. Shortsword of First Blood: I actually like this sword a lot, but I wish it had a +1 enchantment. It's a sword that's best used in the primary hand so that it gets the first strike, but it lacks that +1, making it less likely to hit, and thus not as useful in the primary hand. This always leaves me feeling a little disappointed, and I inevitably decide not to use it.
8. Blooded Greataxe: meh, another one of those weapons that only works when the target has less than 50% of their hit points. Things like this are better in theory than they actually turn out being in practice.
9. Ritual Staff: you get this from goblins, and it looks cool. But it's just a cool looking quarterstaff with a cool name. I'm not saying there's anything wrong with that, it's just that it gets your hopes up when you first see it, and then you're quickly disappointed because it doesn't live up to your expectation.
10. Dragon's Grasp: it deals extra damage to targets that are already burning. Kind of neat, I guess, but it's such a rare thing to be able to take advantage of. I suppose you could work it into some sort of strategy, but all of that seems like more effort than it's worth.
11. Sword of Screams: I love Nere as a villain, and I love the look of this sword, and the name of it is ... wow, what a great name, and then I get it, and... it's not terrible. It doesn't say so in the listing, but it does an extra 1d4 in psychic damage, and that's okay. But... a part of me was hoping this sword was what gave Nere the "Shield of Screams" ability he displays in combat.
12. Shields, any of them: not a single shield with a +1 anywhere? You'd think there'd be at least one somewhere; in fact, Minthara's Iron Banded Shield is listed as a +1 on the overall wiki page, but it's not. It's just a standard shield.
13. Simple Robes: so many of these NPCs look good, and then you take their loot and get simple robes that look like generic simple robes. I'd like some more interesting looks, and I keep feeling disappointed. I get that this is early access, but there's a lot to be said for how the outfits look.
14. Dark Justiciar Helm: I was hoping this would look like the helmet of Shar that's found in the Blighted Village, only that it would have enhanced properties (and maybe a dark jewel embedded within). So I was disappointed in the look of it, to start. But I was also disappointed when I realized that it only gave a +1 to saves in limited circumstances. Which I guess is better than nothing, and it also protects against crits, which is great. Still. I don't know. I guess I want Shadowheart to wear it, for flavor, which means I want it to look great *and* I want the benefit to clearly outweigh other potential choices for her, like Wapira's Crown, for instance.
15. Dark Justiciar Mask: this is another item I really want to consider for Shadowheart, but a +1 to intimidate isn't enticing enough for a piece of head gear. I can understand how some players might mix and match items depending on the circumstances in the game. Like, a player might put the mask on if they know an "intimidation" role is coming, but that's not really how I enjoy playing. I'd much rather find a stable and consistent outfit.
Anyway, I'm sure there are more items, but these are ones that immediately come to mind.
And it's worth mentioning, there are plenty of pieces of armor and weapons that I *do* like. This is just a list of ones I particularly found disappointing for one reason or another.
ETA: Oh, I almost forgot one. Also:
16. Light of Creation: awesome weapon, but it has the chance to stun the wielder? C'mon, isn't that a little harsh? I guess it would be understandable if the chance was something like 5-10%, but from what I gather, it's not. It's more like 25-30%, meaning you're getting stunned every 3 to 4 times you use the weapon. That's a lot. It basically makes this weapon unusable, removing it from any reasonable consideration. But consider, if it only had a chance to stun the wielder every 10 to 20 hits, well, I could see someone being more likely to take that chance and enjoy the flavor of the weapon.
But I'd also like to add the shattered flail and the OP "when you heal" items.
