It's reassuring to we that write that even Shakespeare can make something that just isn't any good. While I've heard bad things about plays like A Winter's Tale, I haven't read them yet, and can't honestly comment on them. What I can say, though, is that The Two Gentlemen of Verona is an extremely poor play.
I sincerely believe that Shakespeare must have been facing a deadline and finished the play in a panic. It's a shorter play, and it spends much more time than a regular story establishing the basic problem: three men are in love with the same woman.
To understand the suddenness of the ending, I have to explain the basic idea, so bear with me...
Valentine and Sylvia are in love. Proteus used to love Julia, but has fallen for Sylvia. Julia still loves Proteus, and follows him around dressed as a boy/manservant. Thurio, the son of the Duke, is courting Sylvia, and the Duke banishes Valentine for showing interest in the woman he wants his son to marry.
The ending goes like this, and I am not exaggerating...
All the major players are together
Julia: I'm not a boy, I'm your former lover!
Proteus: Ah, I remember you! I suppose I'll fall in love with you again now.
Sylvia and Valentine: We still love each other.
The Duke and Thurio: Well, why didn't you say so! We're no longer interested in Sylvia, and we'll un-banish Valentine and his gang of murderers and thieves.
Nothing really happens to justify these changes. It's as though he wrote himself into a corner, had to finish the play, and just said, "Screw it! Everyone lives happily ever after!"
Shakespeare Count: 7 down, 30 to go.
Next Up: Love's Labours Lost
Now playing: Irene Cara - Flashdance