You definitely need tankiness on yi, especially since he is a close ranged champion. Also, your guide lacks BB coding and so makes it boring for the reader to read. No one wants to read a wall of text. There are a few guides on mobafire that will help you with this, but for now it's a -1 from me.

You should mention Armor Penetration for example by mentioning Last Whisper.
For coding what was criticized by Bligen: http://www.mobafire.com/league-of-legends/build/making-a-guide-101506

Okay, I'm over this whole entire BoRK thing I had this week. This guide is still completely deserving of a downvote regardless.

ok just from the name - this will suck, yi is a melee champion so he needs to have a little bit of tankyness...
build + runes + masteries = 8% + 26 armor pen... so if i get a frozen heart + sunfire you're gonna **** yourself and do no dmg
items - no situational items AT ALL, just because this worked once doesn't mean it will always work
Perseverance is a useless mastery
the guide itself has no bb coding whatsoever and is a huge wall of text....
just archive it

Perseverance is bad, lack of formatting.