First of all: this guide is for AD Nidalee only! There are plenty of other guides here for AP Nid, and if you want to chuck spears all day feel free to get your build somewhere else... it's not my damn problem when someone dodges it though. Which brings me to the meat of this chapter...

Why AD Nidalee instead of AP? I mean she is meant to be built that way, yes? Her champion page has her labeled as a support and 5 out of her 6 skills scale with AP, so how can you justify playing her AD?

That's a good question. Very smart of you to ask that... here are the top 5 reasons how I justify playing AD Nidalee over AP.

1. AP Nidalee has only one solid source of damage during a teamfight, which has 5second cooldown and can very easily be dodged.
2. A nidalee that is in cougar form for a whole teamfight takes less damage via increased armor+MR, and deals more even, consistent damage, as she is never more 3 seconds away from her next damaging attack (pounce)
3. Nidalee's kit is too good to deal damage only with the spear. Most (all except for spear) of nidalee's abilities are close range, where an AP nidalee is likely to be torn to shreds in seconds. A tanky AD nidalee does not have this problem
4. The psychological factor: When you have a super-beefy nidalee you can outrun the entire other team in a chase for a good two minutes, and live. This not only can be vital time wasted while your team revives, but is also incredibly demoralizing to the other team.
5. It's easy to transition from an AP only nidalee player to AD. You can still get spear kills without any AP... sure they won't be the 1000-dmg loaded spears that carrys have come to fear from AP Nid, but as you mature in your nidalee playing career you realize that the spear is not the be-all-and-end-all of Nidalee's power, but is just one small tool in a roflstomping 7-ability kit.

tl;dr: Spear is an unreliable source of dmg during a teamfight, AP nidalee is too squishy to execute her kit correctly, AD nidalee is far superior in every aspect, etc.

Flat AD quints and marks to help with farming and harassing, and flat armor seals and MR glyphs to help with early damage mitigation

tl;dr: +15 dmg, +13 armor, +12 MR

Executioner is an absolute necesity on AD nid. You can make up for the lost tankiness in the defense tree with items, you cannot make up for the 5% stacking bonus that synergizes perfectly with takedown. Cooldown reduction in the first tier to get the CD on pounce lower. Last hitting on AD nidalee is easy, so havoc over butcher to help with cougar Q burst DMG, as well as a point in demolitioner, cuz when you get the spears pumping with heal buff you can melt towers by yourself SO FAST, you're basically a 2nd carry. It's honestly just my personal preference though, because the way I see it champ kills are nice but at the end of the day destroying buildings is what wins games. Also chose not to put any points into Lethality/Frenzy because if you think about it, even if you crit for 1000, that talent is doing an extra 50 damage. That's a dog turd's worth of damage imo as well as the 10% AS on crit. It could work for an ADC with a really high crit chance, but it doesn't compare to brute force and sunder for AD nid.
After offense tree, I choose to go defense. +30 HP, +5 armor and +2 MR to help vulnerable pre-level-6 nidalee with early trading? Yes please... When that darius ignites you, you have 1 HP pot and no mana left and you escape with 12 HP, you'll be thanking your veteran's scars :P

tl;dr: 21-9-0