^ To be fair, it's not just sci-fi fans. I think a lot of people who really like a novel, of any genre, will have a cinematic version of that novel in their head. When a cast deviates from that, fans, who can be quite possessive of a property, get upset. Witness the reaction to Cliff Richard as Heathcliff (in his stage version), from fans of Wuthering Heights or from 007 fans to Daniel Craig as James Bond. Guess which one managed to overcome superficial dissimilarities with a compelling performance?
Sci-fi and comic fans don't have identical views but have been welcoming of many performances or castings where the actor doesn't necesarily resemble the original depiction. Ian McKellen as Magneto, Heath Ledger as The Joker, Liam Neeson as R'as Al Ghul, RDJ as Iron Man or Chris Evans as The Human Torch come to mind. None of them look exactly - or in some cases at all - like the way their characters were depicted on the pages, but their performances were generally well received.
Part of Lestat's attraction is that he's an ageless, eternally youthful vampire. RDJ is a wello-lived in 40 something, looking every one fof his years. It's part of the same reason why James Marsters has said he can no longer reprise the role of Spike. I do think Downey is a great actor, one of the most watchable leading men around but I just don't see him having Lestat's persona. He's quirky, funny, likeable and offbeat. Lestat is effete, preening, occasionally childlike but frequently dangerous.
At the end of the day, if you're going to ignore the appearance and characteristics of the character you're adopting for the screen, why bother calling him/ her after that character? Why not invent your own?!