[akhtene]

I can think of two explanations to that. It seems to me that the Lord of the Rings was written from the human (or shall i say mortals') point of view, so the elves are portrayed as an ancient, mystirious, somewhat superior race. The Silmarillion is more like elven cronicles and probably describes things as elves saw them.
Another thought, The Silmarillion describes the 'childhood and youth' of elven race (if it's possible to say so). The childhood -the time when elves were invited to Valinor, taught and cared for there. Youth - rebelling against all authorities, desire to do what they thought right, gaining experience and making mistakes (often deadly). And just because they were immortal they accumulated life experience and wisdom, it was not lost with every next generation, as it happens with men when only the wise learn on someone else's mistakes. And by the time of the Lord of the Rings the wicked had probably had enough time to get what they deserved or to repent and change their ways.
So I believe immortality (not just physiological, but ability to accumulate knowledge) is what made elves so different from men. And I suppose the same immortality eventually makes elves 'weary of the world' and so at some distance from it. Maybe the two races were not meant to be so differnt from the beginning, it just happened with time.