I'd be all for a dedicated animated movie thread. Hand drawn animation especially is something I've long loved and will truly lament once its gone for good. In the west that day is probably already come and gone so thank goodness for the Japanese! Since you listed Grave I take it you're open to Japanese Anime in general. Studio Ghibli is the natural starting point and I'm sure you've heard of some/many of their films (Grave being one of them). Princess Mononoke, Spirited Away, Howl's Moving Castle, Porco Rosso, My Neighbor Totoro, Castle in the Sky, Kiki's Delivery Service, Nausicaä of the Valley of the Wind -- all being beautifully animated, well done and well told stories. Some more whimsical than others, and all of them really having some mature themes within. Really, Ghibli and Miyazaki's films are the preeminent examples of Japanese animation excellence.
Still a great many other films to consider too:
Cowboy Bebop: The Movie
Akira
Wings of Honneamise
Armitage III
Steamboy
Ninja Scroll
Vampire Hunter D
Ghost in the Shell (and it's sequels)
Millenium Actress
Metropolis
Paprika
Summer Wars
5 Centimeter per Second
Tekkonkinkreet
Patlabor
Tokyo Godfathers
Jin-Roh: The Wolf Brigade
....and a million more I'm sure I'm forgetting.
Out of these above Akira, Ghost in the Shell, and Wings of Honneamise had huge impacts on me as a kid watching them. I count Paprika among my favorite movies ever as well, just incredible and a terrible shame that Satoshi Kon died so young.
This isn't even counting any animated series'. Tons of good ones to choose from there too. My faves: Cowboy Bebop, Ghost in the Shell: Stand Alone Complex, Last Exile, Paranoia Agent, Samurai Champloo, Fooly Cooly (FLCL), and finally The Big O. That there barely scratches the surface of the thousands upon thousands that are out there both good and bad, but I generally find it harder to get into watching too many different series'. Who has the time?
Ok, so that's Japan. Other countries do the animated thing too:
Heavy Metal
Persepolis
Waltz with Bashir
The Triplets of Belleville
The Secret of Kells
The Illusionist
A Cat in Paris
Song of the Sea
My Life as a Zucchini
The Red Turtle
(I've yet to see these last two, but I'm including them as I've read they are very poignant)
Only thing I would add to these, and since you listed Coraline earlier, would be a few clay or model animated films, mostly from Nick Park and the people at Aardman Animations. All of the Wallace and Gromit shorts are great and the feature Wallace & Gromit: The Curse of the Were-Rabbit is tremendous as well. I know you said more "heavy stuff" but really they are quite awesome. Chicken Run was pretty good too, and Early Man looks to be hilarious coming soon. The folks over at Laika (which is run by Phil Knight's son) having been pretty solid too, with Coraline, The Boxtrolls, ParaNorman, and Kubo and the Two Strings.