Maybe it doesn't have enough car chases or explosions or other shiny things to keep your attention. People seem to like movies that are very loud and where bright things happen on screen a lot these days. Citizen Kane doesn't have much of that.
I love Citizen Kane - it was revolutionary at the time, and is still better than 99% of movies made after it. I love how we're looking at the life of this larger-than-life man, but we're never seeing anything as it actually happened (except when the reporter is interviewing people that knew Kane, I suppose - and even then it's his POV) - the entire movie is told from various points of view.
Technically of course it's a major achievement. It's like the movie equivalent of landing a man on the moon. There are so many firsts in Citizen Kane, and so many shots were new that it redefined filmmaking more than it enthralled audiences at the time it was released. That just about every film post-Kane borrows from it in some way means that you've seen Citizen Kane many many times before - little pieces of it in just about every other film you've ever seen.
I can understand how it may not seem so fresh and exciting after 60+ years of homage.
Rosebud is of course a Macguffin. Sure, we find out who Rosebud is (no spoilers here for the cultural ostriches out there who don't know) but is that really the central part of the story? It sure does explain a lot about Kane, and what he was striving for his entire life.
I've seen Citizen Kane at least a hundred times, at one point going through it in slo-mo with a film group so we could discuss it frame-by-frame if need be. It took us three four-hour nights to finish the film, and then we spent the following week discussing it.
However, if I had to list the most enjoyable "great" film I'd have to go with Casablanca. There is nothing that isn't in Casablanca. Guns, sex, music, dames, intrigue, heartbreak, love, death - it's an encyclopaedia. And it moves really fast so modern (American) audiences who have grown accustomed to bright flashes of light and loud noises can somewhat enjoy it too.
P.S. to Wombat: I am sorry that you did not enjoy the Film Noir film festival. If you've followed my list you've probably seen some of the best the genre has to offer. These films were made in the epoch of the Cold War, as the world was changing, new alliances were being formed, and everyone was suspicious of their neighbours' motives. I think Film Noir translates fairly well to our Patriot Act smoke-the-terrorists-out-of-their-holes era, but again, they're slower-moving movies with less action in them, made for an audience who was expected to pay attention.
EDIT: I know the above sounded like I was calling you "simple" or "slow" if you didn't like some of these "great" films - that isn't the case at all - I was just pointing out how different they are from most of the crap being shoved out lately, and how bad movies train people to watch more bad movies. It's pandemic.