I wouldn't say a downward curve Speedy, but a change of emphasis.
GTA3 on PS2 was perfect. Fantastic new 3D world etc etc, but not only that it was HYSTERICAL. Great characters, good plot, funny missions. Vice City was exactly the same. San Andreas was a bit darker but the characters were likeable enough and the missions were generally brilliant.
Enter the PSP. Liberty City Stories was, again, hysterically funny. Vice City Stories was poor. Technically excellent, but the story was dismally dark and depressing. I play GTA games to laugh and have fun, not mope.
Which brings me onto GTA4. Technically it could legitimately claim to be the game of the decade and I wouldn't begrude it that. Technologically it is an outstanding achievement. But where's the fun? Taking someone out for a date isn't fun. Choosing to let someone live or die isn't fun. Flying remote controlled helicopters into builders wielding spanners is fun. Shooting someone's head clean off and blood spurting out of their head is fun.
Where's the fun in GTA4? There is none. It's stony faced serious. There's occasional comedy relief from Roman and Bernie, but it's short lived. VERY short lived. I'm told TLATD is a bit more light hearted. I hope so. GTA: Chinatown Wars is a masterpiece of a game. Witty, funny and varied missions. But it's not sold well. I fear seriousness is here to stay.