This new Saints Row 2 video is a nice summary of the main point I was making in my (admittedly piss-poor) initial impressions review of GTA IV. My argument was that Rockstar has veered so far from the original flavor of the GTA series that now the Saints Row series is more GTA than GTA. That original flavor was wild, free-form gameplay and downright silly shenanigans. The original tone and over-the-top gameplay of the series seems to have been slowly fading, to be replaced by different values.
I absolutely have no problem with developers changing style. People change. Interests change. And GTA IV is certainly a financially successful game that pleases many gamers. Rockstar did well.
But I do believe any series should remain faithful to the expectations of its audience... faithful to the basic style that defined the first product; for ethical reasons as much as marketing reasons. If you're interested in making a new sort of game, don't realize it by changing an established IP. Start a new IP. If you establish your company as a respectable brand (Blizzard, Bioware, etc), then you can use that to about the same effect as a game brand. Command & Conquer is another series that has wandered quite a bit, though at least they eventually divided it into subcategories (Red Alert series, Tiberium series, etc).
In this case, I think the GTA series has wandered too far from its heart. I sold my copy of GTA IV to a friend.