I've had R10's and L3000's for a year and a half, and love both of them.
The biggest way I would contrast them is by saying that R10's are a first impression headphone, and L3000's are not.
Here's what I mean ...
The first listening experience one has with R10's, given a good recording, source, and amp, will usually evoke a Wow! response. Hearing that enormous soundstage, incredible detail, and overwhelmingly musical presentation for the first time is an experience that remains with most of us. Part of the magic of R10's is that the Wow! experience keeps repeating itself with every listening session. R10 world is a special place that one can return to time and again, and be rewarded with pleasure and joy.
On first listen to L3000's, again assuming good recording, source, and amplification, most will respond with, "these are very nice, but not amazing." I know I did, and even questioned buying them. L3000's are a more subtle experience that take time to fully appreciate (at least for this unsophisticated listener). I recall that I did not care for coffee or scotch the first time I tasted them, too
As time went by, and both cans broke in, the L3000's blossomed into an exquisite flower. I would find myself listening to a piece with the L3000's and sudddenly feeling, "wow, these are incredibly good." I've come to appreciate the L3000's every bit as much as the R10's. In many ways, they are more balanced and honest in their presentation. Where I used to reach for the R10's 80% of the time, it is now evenly split. There has been more than one occaision where I've felt the L3000 is the better headphone.
I realize that this discussion is light on actual description and analytical examples, but I think one's emotional response to what our equipment brings to us is every bit as valuable in evaluating the gear.