I've been playing Minecraft for years and I still see the same mistakes everywhere. Here's a quick breakdown:

1. Iron farms — Most tutorials skip the villager line-of-sight trick. If your golems aren't spawning consistently, your villagers can't see the zombie. Fix that and you'll 3x your output overnight.

2. Creeper farms — If you're building above Y=200 and wondering why rates are bad, it's because you forgot to light up the caves below. Mob cap doesn't care about your build height.

3. XP farms — Enderman farms are great but most people build the platform too small. You want at least a 43x43 spawning pad. Anything less is leaving XP on the table.

4. Sugarcane — Zero-tick got patched ages ago but a properly designed observer-based farm still produces more than you'll ever need. Most builds waste space with unnecessary redstone.

5. Villager breeders — Bed placement matters more than food. If your breeder stopped working, count your beds, not your carrots.

I break down stuff like this every week inside NetherNet. If you're serious about optimizing your world, come through.