Upon consideration, I've reached a conclusion.

Some items that are in the game right now feel unsatisfying to play with/against.

Some champions in the game feel unsatisfying to play with/against.

I think I've found out why, and that it's that a lot of damage in the game right now isn't easily visible/telegraphed.

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Duskblade and Stormrazor are two items that are looked down upon because they bring either free damage, are super broken on a select few champions, both, or are a band-aid/mandatory item.

Both of these are AD, if you'll notice. The reason the boards hates on them so much is because when you're facing a Duskblade user, that extra damage hurts.

So why are AP mid laners doing well?

That's because Riot decided to keep AP values high, but instead of adding interesting damage gimmicks, except for Liandry/Spellbinder, most items are just raw stats or Luden's Echo. Luden's Echo is a damage amplifier that is highly telegraphed, because it only works when a spell lands. However, when Assassins have mobility, the complaint is that Assassins can reliably proc Duskblade/Stormrazor without actually using "skill".

This creates an imbalance, where a few AP champions are overwhelming, and a few AD champions are overwhelming.

The few AP champions that are overwhelming scale well with AP, and take advantage of high AP ratios and high AP statsticks to dish out heavy damage throughout a fight.

The few AD champions that are overwhelming rely more on the massive amount of "free damage", like Electrocute, Duskblade, and Ignite, which is why high elo/professional players don't really struggle against assassins. Assassins rely too heavily on runes and those free damage procs to actually be useful. Champions like Talon rely on roaming power instead. On the off chance they can stat combat, it's usually a matchup of "Do I have enough Lethality/Armor Pen and AD + AD ratios + Ignite + Duskblade to actually pop this ADC?" Assassins become hugely unfun to play because it's literally just a matchup of stats, and an ADC near a support just won't die because an Assassin can't actually reach the ADC in time.

High elo players can also easily notice what items will do how much damage, and can easily play around Duskblade / Ignite damage values, and these are telegraphed by the enemy champions' position. If you're in AA range, you're going to get hit. That's how assassins work right now, and that's why higher elo players find it so much easier to counter.

AP champions have a different problem. They are also huge stat sticks that rely on gross AP ratios and gross AP values to dish out damage.

Let's use Zoe to explain. My first impression of Zoe was a power creeped version of Lux. This doesn't make too much sense, so let me explain. Zoe has an easy to aim E, which lets her run E into Q, but Lux relies on hitting fringe Qs to set up her full combo. Why are they similar? Zoe's trade pattern early game is similar to Lux's, where she pokes repeatedly with Q > AA > Q > AA, vs Lux, which is E (> AA). Both harass repeatedly until they can land a killing blow or chain damage to death with help of cc, but Zoe's is easier to land, and augments her damage more. However, Lux is better in teamfights, while Zoe is better in a 1v1, because of the hitboxes of the damage. Huge laser vs tiny star. However, as I played both champions, I realized that really what made Zoe seem so frustrating was that her damage was so badly telegraphed. If you get hit by E, but you don't see Zoe, she'll suddenly jump at you with a tiny star dealing 90% of your max hp. Tack on Ignite and you're dead. Lux has clearly visible skillshots that show you what you're about to expect.

Looking at the top AP and AD champions right now in high elo, we have Kassadin, Ahri, TF, Malzahar, Orianna, and Anivia as the top AP champs with Irelia, Yasuo, and Talon as the top AD champions. Other than Twisted Fate, Anivia, and Orianna, all other champions have a "kill zone" that's greater than just AA range. For Yasuo, Irelia, and Talon, they rely heavily on the same mechanics of other AD champs, but they have an edge on how easy it is to get the enemy INTO AA range to deal all that hidden damage. Kassadin gets R that deals a lot of damage, Ahri gets her R and also E's damage amplifier (which was a mistake), and Malzahar has a big range around him called "Do not walk in or I will ult you".

Orianna relies on her Ball, which is highly telegraphed. She's good for teamfights. Anivia's Q is the slowest projectile in the game, and you can literally walk out of it with ease. The changes to her R mean it won't deal damage until it's been telegraphed. TF is good because his roaming potential is so good, because it's instant. These three champions utilize heavy heavy power through OTHER means that aren't "Oneshot fiesta". These three champions are what I consider "Healthy" mid-laners, which are essentially play-enablers/teamfight champions.

Every other champ is "Hey let's rush at each other once we're in range, see who has more hidden damage packed into their kit and items, and they'll be the winner. If I dodge your skillshot I get an extra second of DPS/x damage negated." This is why it's so unfun to play Midlane right now.

Even champions like Yasuo, who look like they're having flashy plays, are mostly just 1v1ing multiple champs, repeatedly, in a short interval of time. Champions like Orianna and Anivia rely on being able to actively fight multiple enemies at once at slightly less power. Twisted Fate, Orianna, and Anivia also have highly telegraphed DAMAGE, not just highly telegraphed location of damage, which is huge, because everyone else relies on hidden INDICATORS of damage as WELL as hidden damage.

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TL;DR: Hidden damage is ruining midlane and is making it unfun. AD champions tend to have items that hide damage, while AP champions tend to have an unclear range of where they can deal damage from, and how they can deal it.

Disclaimer: I'm tired so I probably left out some key points but oh well just ask me and I'll try to answer.