Actually, Halo 3 does have more enemies, more encounters, different weapon distribution (the power weapons, and even things like grenades, show up sooner in the campaign on heroic and legendary) and generally more "stuff" on higher difficulty levels.But for me that just means there is a lack of SP content because a harder difficulty doesnt give you anymore content.
Also, i don't get the people bitching about how games should "never score a ten". If you're not using the full range of your scoring system, then WTF is the point? If you never give 10s out, even for the very best games available, you've got a practical scoring range of 1-9, not 1-10. And then you might as well get rid of the 10 rating anyway, as you're never going to use it, and then you've got 9 as your 'perfect' score and you have the same problem again.
Same goes (in the opposite direction) for sites/publications that never score anything below a 5 - why have 1-10 if you only use 5-9? At that point you mgiht as well give up the pretense and switch to a 1-5 system (although if they did that, they'd probably never give out scores below a 3...)