well sure, but why not use the 300mm f2.8 on a crop camera (canon do make them) such as a 7D or even 1D. In which case the comparison will not be as disparate as you are making it by forcing the comparison with full frame.
Cilacz mostly answered that for me, but again: the 300/2.8 of any brand are big, expensive and heavy. A 200/2.0 on DX looks like a better option, being just around 200mm long (the 150/2 is 150mm) against the 260-270mm of the 300mm lenses, but they are still nearly 3 kilograms, nearly twice as much as the 1.6 kilos of the Zuiko. And they are both around $6,000, like their bigger 300mm brothers.
Currently, I use Nikon cameras with an 80-200 f/2.8 AF-S, an excellent lens which takes me to the 300mm eqv. that I need. But it's old and doesn't have VR, and the short end is too long, at 120mm eqv. So I would need another zoom, like the Tokina 50-135/2.8, to fill the gap between the normal zoom and the long one. While there are ways to solve this, none do it as elegantly as Olympus, which is natural, since the lenses are designed from the ground up with the smaller sensor in mind.
I also have a Nikkor 300mm f/4.0. It's a superb lens, but too long for most of what I do when used on DX. It's mostly in the bag or at home. Should I need the 400mm eqv., the 150/2 is still rather good even with the 1.4x TC.
Finally, there's the headache with the normal zoom. The only ones that really impress me, both when I look at test results and real life photography, are the Zeiss/Sony 24-70 and Zuiko 14-35. Some of the others, like the Nikon 24-70, are good, but the Zeiss and Zuiko are better. They are also a bit shorter, which make them easier to hold.
The length of lenses is something that doesn't get discussed too much, but when I'm running around with a camera and lens the whole day, there's a lot of difference between a short and a long lens. That's basic physics, and no big surprise. Here are one of the disadvantages with the Olympus system also: The 35-100/2.0 is rather front heavy. Totally, it's about the same weight as the Nikkor 80-200, but it feels a bit heavier.
Nothing against Canon owners btw. There's no such thing as an unbiased opinion of course, but reading between the lines is one of the challenges being on a forum like this. As for me, I have considered Canon a number of times, and had actually decided to buy the 20D as my first DSLR. What stops me every time is partly ergonomics and partly the white lenses. Many of the places I travel, I don't need a white lens indicating that I have loads of money. Brand recognition has its negative sides too, and the white lenses of Canon are almost as famous as Coca-Cola.
Ooooh... many words. Hope they make sense
