Just curious, why aren't there any in-game items which allow attacks to deplete the target's mana? Such an item would help to combat AP powerhouses, add a new element of strategy to the game, and even increase the importance of Magic Resist, were it to reduce the effect. This could even build into an item that grants Mana Vamp. As it stands, I know of no items or even abilities that can cause this effect.

wits end used to do it, but it was stupidly broken against people with low mana pools

You're talking about an earlier incarnation of wit's end.

Thing was it hurt bruisers and tanks with low mana pools more than it hurt mages. (maybe we should bring it back, lol)

i think their main argument was that it was ' ANTI FUN '.

Ooooh. How did it scale? Percentage of damage done, like with Life Steal, etc? Why not have made it something like 3% of target's max mana? Or even remaining mana? And if it needed further nerf, it could have had a short cooldown on its effect, like Manamune.

It was like the current wits end I think, it just did a flat amount of mana damage. But the thing is, most casters will build a decent sized mana pool or if not, will get blue buff. By the time you could afford the wits end, the caster would have enough of a pool that it wouldn't really cripple them. The ones it really hurt were bruisers/tankydps whose mana pools are pretty small, making them useless if they were skill(qwer) dependent.

Oh. So do you think that having it deal a % of total mana pool on strike would have made it more feasible, since it would have less effect on smaller mana pools? Just seems like it would be a nice utility option for team battles, since well-built APs can seemingly never run out of mana. For that matter, there are no items or abilities which affect the target's mana regen either, are there?

Mana burn was removed because, yes, it was decided to be anti-fun. It caused major problems because either the ability was worthless (i.e. the caster still had enough mana for their spells) or it was a perma-silence (because they were out of mana). It also was decidedly less effective against the numerous manaless champions.

It may have been a cool effect and such, but again, either it doesn't remove enough mana, and you may as well have bought an item that has Lifesteal or extra on-hit damage, or it will remove enough mana and make one person on their team unable to cast spells for the rest of the team fight (unless they have Chalice+Blue).

Bummer. It just seems like it would be a cool effect, and very useful for slowing down those Veigar Nukes. Seems like it adds to the difficulty of encouraging variety in item builds when so many of the items in the game do more or less the same thing, with varying degrees of success. As far as I am aware, very few champs even have a silence ability, and it is usually only a second long, and on a 15 second cooldown. Are there any items in the game which offer on-hit silence?

Talon's matches that description, however there are many others. Specifically...

Blitzcrank's is almost not worth mentioning, as it only lasts 0.5 seconds, but it's a massive AoE burst too.
Cho'gath has a cone silence that lasts 2-3 seconds and has a relatively short CD.
Fiddlesticks has a bouncing silence projectile, 1.2 seconds after the last hit to touch the target.
Garen's main engagement tool has a 2 second silence attached to it.
Kassadin's core skill is an almost 3 second silence nuke on a short CD.
Leblanc's main burst is ALSO a short CD almost-3-second silence nuke.
Lulu's Polymorph technically silences and disarms your auto attacks for 2.5 seconds?
Malzahar has a wide skill shot nuke with a 3 second silence attached.
Soraka has a FREE Silence nuke that lasts 2.5 seconds.
Viktor's ultimate is like Blitz's, 0.5 second silence but massive AoE damage.
No, because Riot decided not to put any hard disabling abilities on items. Some champions are balanced around not having hard CC (such as Shyvana and Katarina), and would become rather powerful if they could gain access to such abilities. Stun the person that's saving their disable for Kat, ult the rest of the team, carry team fights even though you're an assassin and all that.