Why does Talon counter mid champions?
Solution 1:
A Talon vs an AP champion looks pretty good on paper. Let's break it down.
Abilities
W Rake: Deals AOE physical damage to a wide area in front of him and slows enemies. The popular AP mids have good creep clearing abilities that could push a traditional ranged AD back to the tower, which then allows the AP champ to gank side lanes. Rake checks this. Although Talon is melee, Rake allows him to safely clear the creep wave when needed. The slow is also nice for ganking and chasing.
E Cutthroat: Instantly appears behind a unit and silences it for 1 second and amplifies subsequent damage dealt to it. This is the key for applying pressures on AP champs. A silenced AP champ feels like a sitting duck and an inexperienced player will try to run, giving Talon a free auto-attack. Cutthroat also resets the auto-attack timer, so you can actually hit twice. Although the silence can be blocked by spell shields, the ability is instantaneous, which makes it very difficult to react against it.
R Shadow Assault: Becomes invisible for 2.5 seconds + 40% movement speed buff (Ghost is only 35% with the mastery); his blades deal physical damage in an area around him. Great ult for a mid laner to have. It makes him extremely difficult to gank, gives him a great chasing mechanic and is a great ganking ability. Plus that 2.07 bonus AD ratio if you Cutthroat them first - ouch.
Stats
Magic Resist: Talon has scaling magic resist. Nice. Most AP mids don't have this extra defense (usually melee champions have scaling MR).
Health Regen: Talon has ~150% the HP regen of AP mids. It'll take much more to harass him out of lane and an AP champ would rather be using that mana to farm.
Movement Speed: Few AP mids (I think only Katarina) have such high base movement speed. This make Talon great at dodging skill shots and ganking side lanes.
Health: He starts off with about an auto-attack's more health than most AP mids. It's a small but comforting advantage in the early levels.
Pick
Talon is rarely picked in ranked games. There's either a more nuanced explanation for his weakness in other parts of the game, or he's just underappreciated. On paper, he looks very strong with a Maw of Malmortius.
Solution 2:
Talon's design is based on countering AP characters, who themselves are frequently midding. He has an insta-gap closer (Noxian Diplomacy) that also silences, and while it's not a long silence it's long enough for him to drop a large burst of physical damage on enemy champs. He has a very good slow.
Morgana's Black Shield only blocks magic damage. While it also blocks disables, I believe it's hard to use to counter Talon's Noxian Diplomacy.
Solution 3:
Historically, the best counters to an AP Burster have been AD Casters, such as Talon, Pantheon, and at times even Lee Sin. The reason for this is that most Mids get MR runes and masteries to better deal with their counterpart, and as such have next to no resistance to AD, and as such when they're slowed and silenced (Talon) or stunned (Pantheon) or slowed and kicked around the lane (Lee Sin) they have very little time to react and try to burst down this menace.