Let me preface this by saying that while I'm a huge fan of the original Metal Gear Solid, I only made it about half-way through the second one (I was stymied by the boss 'Vamp') and haven't yet played MGS3 (though I've seen much of it).
MGS4 is a great game so far. The graphics are amazing, even though my PS3 is (1) hooked up to only a 27" SDTV and (2) using only composite video [the 360 has component cable priority currently]. Even more so than COD4, the game begins and plays much of the way through in a very cinematic fashion-- though in some ways there may be a few too many cut scenes.
A couple gripes so far:
Automatic weapons aren't necessarily. I don't know if it's just because the weapons stop firing when Snake gets hit or the controls are sticky, but it's quite difficult to spray down some enemies with suppressing fire.
Why do I need to press two buttons to aim down the sight? Granted, the Metal Gear games have never been first-person shooters (and only the last two have even allowed you to shoot from first-person), but when you have a rather long combat sequence against 20-30 enemies while escaping a building, it's annoying to hold one button, press another to aim down the sight, and then press yet another to fire--every time!
Compared to MGS and MGS2 (don't know yet about MGS3, obviously), hand-to-hand combat blows. Maybe it's because there are too many nuances introduced that weren't there in the first games, but whereas I was pretty good at throwing (and especially choking/breaking necks) in MGS, I suck at CQC (close-quarters combat) in MGS4. Suck may not actually be strong enough. I pretty much get my ass handed to me every time I try to throw someone (partly because I hate CQC being mapped to the R1 button.
Which opens up another gripe--when the L2 and R2 buttons are the most trigger like (though not nearly as comfortable as the X360 controller), why are weapon commands mapped to L1/R1 instead? Wtf?
Which opens yet another grip, more about consoles in general. I'm not sure if PC games are still so flexible, but pretty much every PC game I've played allows you to completely remap the controls if you desire. And, given the hard-drives on the next-gen consoles--especially the PS3, where game software is regularly installed into the OS--why can't developers offer the flexibility to remap buttons. While I admit most control schemes are pretty well laid out, there are some (MGS4 in particular) that I really wish I could customize.
This doesn't mean I don't like the game. It's pretty good. I just think there are some definite improvements that could be made.