I was wondering why AP shen is bad. His R scales off of AP for 135% of AP. Imagine if you have 200 AP your shield will be a 1000 if it was maxed rank. His W procs with his Passive so each time you get someone it goes down by 2 seconds each time you auto. His Q does 60% AP and that can lead to a lot of damage. His shadow dash is 50% AP. He can be sort of a more damage tank... Can't he?
TY for the 1000 shield Shen!
The big thing is his range. If his Q had about double it's current range he could be super squishy as an AP champ and still be effective, however as it is he simply dies too fast. I had a lot of fun with him in URF mode as a full on mage, but that's only because he was artificially tanky due to spamming his W.
I main Shen and I played almost 900 ranked with him as my main top, support and jungler. I few occasion (trollers) I've also played him mid with decent results. Said that it's not he's not viable but if you want to deal a lot of damage you have to focus on too many things. First of all you can only buy AP items that gives no mana but a lot of HP because of your passive and Q. Secondly since your are a melee you still need a lot of AR and MR or you're going to explode in every team fight. In fact with Shen you can't engage/disengage like an assassin. You have to focus on AD and AS too.
In short from my experience a full AP Shen is viable only when your lane opponent is retarded.
You don't do a whole lot, if you wanted to block a ton of damage you could pick AP Kayle who at least has a slow, shred, speed boost, heal, and ranged damage with an ult that can potentially block much much more damage than Shen's ult would even if you had 0 AP. Only thing AP Shen would do better is doing this from where ever he's at where as Kayle has to be need the person she's ulting. Shen is better as a tank or bruiser if you wanted damage.
Its not bad, as far as these things go.
The reason AP tanks aren't very good is because they have a hard time building enough AP without being really squishy. Shen, even with tons of AP and a Lich bane, can't oneshot someone. Then he probably dies before he gets his Q up again.
Shen especially has a few issues. Manaless, can't build the best AP bruiser item in the game, 
RoA, and every CDR item for APs include mana regen in some form. Plus no pushing potential without 
.
That said, the est buildpath for him is lots of AP and Health. 
in no particular order.
Ignore everything what others said.
It's just 2 words: energy costs.
Nothing more.
tanky AP ftw!
i miss the days when your sunfire dmg would scale with your AP :C
Don't mind these idiots just get full CDR, tons of AP and Health and whatever you build, don't forget to throw in a 
.
Max CDR With 300 AP and like 1000 HP:
Ki Strike will do like 170 Q will do 400 magic damage on a 2.1 sec cd
I don't think it's too bad, honestly. I think maybe building him with resists + AP + CDR could be viable (and maybe getting energy regen runes to help you continually spam w and q), effectively using the big shield to make up for having low actual HP. He might be about as tanky as before but with a higher damage output...could be interesting to try out.
Why Is AP Shen Bad?
You basically invest a boatload of gold for almost no return.
A lot of champions have 100% damage-block potential with things like Wind Wall, Counterstrike, Spell Shield, Divine Intervention, Undying Rage, Blade Waltz, Playful/Trickster, Sanguine Pool, Rapel, Stealth. With AP Shen - you essentially waste your entire income turning She's entire kit into 1 such ability those champions get for free so they can build glass-cannon and obliterate things in the ~3 seconds of fame those skills offer.
In Shen's case, those defenses aren't reflexive, instant-cast things on a 10~20 second CD either, AP Shen is mostly tied to a 200/180/160 second CD for that mega-shield burst. And in Shen's case, you aren't killing anything in 3 seconds. Typical QWER is 1000 damage @18 and gets to 2000 damage (enough to OHKO a squish) with 450 AP, at around a 225% AP scaling. Shen's QWER is 410 + 110%.
Anyways - the key thing about optimal builds is what optimization is.
Doing cool stuff at double the price doesn't mean you're not doing cool stuff, that you can't do said cool stuff, or that doing said cool stuff is proof you're using an optimal build.
Optimal and sub-optimal (bad) builds simply offer less power for their cost than an optimal build, and in the case of AP Shen - you're wasting a noticeable percentage of your gold compared to what other builds could offer.
