Well, there is a large amount - mind you, only the 1st and 2nd gen (and a few ones that evolve from 2nd gen like Tangrowth, I believe) can be caught until you beat the Elite Four- the majority of 1st and 2nd Pokemon can be caught before then though (especially thanks to the Safari Zone which made a number of Pokemon only obtainable in Kanto back in GSC now catchable before the Elite Four), and also the PokeWalker as well). IMO there's certainly a decent variety if you look. But anyways... this list here shows all Pokemon NOT obtainable in HG/SS:
http://www.serebii.net/heartgoldsoulsilver/unobtainable.shtml
Feel free to compare with Platinum's unobtainable Pokemon list:
http://www.serebii.net/platinum/missing.shtml
If you look around there, you can see what Pokemon are obtainable in HG/SS exactly via the dex there... don't believe there's much more you need, so going to close.