Toglietemi un dubbio: il mezzo drago senza dadi vita razziali ma solo con livelli di classe non ottiene alcuna arma a soffio, giusto?

No, otterrebbe un'arma a soffio che infligge solo 1d6 danni.

Correct; a half-dragon with no racial HD would have a breath weapon that deals 1d6 points of damage. Not an awful lot, but this was a purposeful design choice because the flavor of a humanoid half-dragon isn't one that we at Paizo particularly like, and it's one that a LOT of our customers have expressed exasperation with. Primarily because back in the 3.5 days, half-dragon was probably THE most overused template of them all. We (Paizo and our readers) mostly got sick of them, causing the half-dragon to go on the LIST for the last 50 or so print issues of Dungeon. That meant that if someone put a half-dragon into an adventure, they had to have a GREAT reason and GREAT background for that half-dragon.

Half-dragons in Pathfinder are intended to skew toward monsters. Things like the dracolisk would be a perfect example, but other creatures with racial HD work well, particularly animals, vermin, magical beasts, and aberrations.

Now... I do understand that dragonpeople are a popular choice for players as PC races. And even though a half-dragon PC or NPC would have a pretty weenie breath weapon, his ability score increases and defense alone should MORE than make up for that fact.

I could certainly see a feat like the following existing though:

Improved Breath Weapon

Your breath weapon gains power as you gain class levels.

Prerequisite: Half-dragon with 0 racial HD, Ability Focus (breath weapon)

Benefit: Your breath weapon now deals 1d6 points of damage per character level (Reflex half; DC 10 + 1/2 your character level + you Con modifier). You can use your breath weapon a number of times per day equal to your Constitution modifier (minimum of 1/day), but must wait 1d4 rounds between each use.

Non mi è chiara una cosa: chi esattamente era esasperato dai Mezzi Draghi Umanoidi? Master che prima li concedono senza modifiche ai giocatori e poi non riescono a gestirli?

Mah.

Anche a me sembra un mostro interessante. Poi che in certi contesti si esagerasse è vero, ricordo proprio un'avventura di Dungeon dove senza motivo la gnocca elfa malvagia di turno TM era anche mezzo-drago senza alcun motivo apparente, ma l'archetipo può avere infiniti e interessantissimi usi dopotutto.