@Joshua
I'm not trying to sell d20 short, no worries...I've seen far too much of what the system is really capable of to think it's a one-trick pony. D&D, now that's a different matter. I agree with you that, if you take the d20 system and mold it to the
game concerned you want to handle with the d20 rules, you will come up with a fitting result. Note that I'm differentiating between game, setting and system for the simple reason that the latter two make up the first, in my opinion. Contrary to what I've been told a few times by now, I believe that a combination of rules system + setting = game flavour.
The thing is, Oriental Adventures was D&D. As it should be. Rokugan d20 was the setting backdrop for that. As such, you get D&D rules on the Rokugan setting background. It certainly makes for an interesting game, it just isn't L5R. There's too much of a difference between the rules that represent how reality works in the respective games. And example? In D&D, attributes are important for skill check resolution at lowest levels, moderately so at mid-level and negligient at high levels, except if you have pushed them to extraordinary scores through magic. In L5R, your attributes define the basis of your success, but don't limit it. They are highly important for skill check resolution all through your career. The same goes for magic. You don't simply get a few meager bonus spells by a high attribute...your element ring score determines how many spells you can cast per day. Two totally different animals.
That's why both games have a completely different, and individual feel. And I dare say that if somebody took a shot at modelling L5R with what tools d20 gives today, compared to what D&D gave 3-4 years ago, we'd probably have a game that's as close to L5R as you can get. But then I wonder why we need to create a new ruleset, and not play with the one already in place. The rules are not harder to understand than a d20 variation on the game is when you're used to D&D.

Or do some people only play an RPG if it's got the d20 logo on the cover? That's the same for a d20 cyberpunk game, by the way. With today's d20 tools, we could probably create a fitting ruleset to run the Shadowrun background and get an equivalent game. But what for? I can't believe any RPGer worth his salt being so biased that he'd gladly wrap his mind around a d20fied Shadowrun game, but claim that learning a new rules system is too time-intensive.
@ruleslawyer
The resolution system of Shadowrun is not really cluttered, or clunky. You roll at least 1d6 to reach a TN between 2 and 10. You have exploding dice, so you can reach that TNs above 6. Boni and penalties always go to the TN, instead of to your dice roll. Not that clunky, is it?
The difference is that, if you have more dice, you have the chance of
additional successes, with those giving you a reward on the effect you wanted to achieve. Which is something hard to model with d20, in my opinion. That's one of the core concepts of Shadowrun, that being better gives you more chances at having a greater effect than the basic one you reach with one success. And your TNs shouldn't go up to "ungodly" numbers either. Between 2 and 12 is about what you should face.