One of the major differences between Let Me In and Let the Right One In is Matt Reeves' decision to include sexuality into the mix. In an interview (sorry I don't have the exact words) Reeves stated that he was interested in the experience of adolescence and the incipient sexual feelings that come with adolescence.
In the case of Tomas Alfredson I do remember his exact words, "no sex please, we're Swedish." Both the Swedish film version and the original novel go to great lengths to establish the relationship between Oskar and Eli as a non-sexual one. There is anxiety in this relationship, but I would describe this as more of a fear of intimacy than anything else, especially on the part of Eli. It is that part of adolescence that Let the Right One In captures so well. Eli both desires and is terrified of intimacy. Think of the deleted exterior scene and the intended sequel where Eli holds Oskar's hand. Oskar is also very shy and awkward with Eli, backing away from close encounters with his new friend. At no time do I see Eli as a sexual being. If Eli personifies certain aspects of human nature, sexuality is not one of them.
Matt Reeves decides to go down a different road and this means that the relationship between Owen and Abby plays out very differently. The scene that sticks in my mind is the one where Owen is playing at peeping Tom with his telescope. (That is a very long telescope Owen has, isn't it
Feel free to disagree with me, but these feelings are not reciprocated in the same way by Abby. Abby is a serial heart breaker. Why should she be anxious about intimacy with another person if she has done this thing over and over again in the past? Abby for me comes across as aggressive, self confidant, larger, and most of all older than her Swedish counterpart. She is the older woman in this relationship, leading on the reluctant Owen who is both attracted to, and terrified by his new friend.

