I have several levels of quality to my tombs.
Soldiers that die below champion/legendary status get buried all together in a long hall - the coffins are all in a row, and the walls are smoothed, with the odd weapon and armour stand here and there. These rooms are continuously updated with more dead dwarves, so theyare kept open.
For normal dwarves (no history of making an artifact, or doing something epic) I have long, 1-tile tunnels with 2-tile rooms branching off of them - each room houses a coffin. Once a dwarf is buried in there, the room is sealed either by a constructed wall, or by a door if I'm getting sick of dwarves walling themselves in.
For pretty good dwarves, like my low-level nobles, dwarves that created an okay-ish artifact (oh look, a masterwork earring. Gosh.), they get a 3x3 room, smoothed walls, one or 2 decorations and walled up/locked in upon death.
Champions that die, or dwarves that die doing something epic (my unassuming cheesemaker that takes down 5 goblins before dying when caught outside) get a 3x3 room, with engravings on the back wall, a statue, perhaps a coffer for the ordinary dwarves and an armour or weapon stand for themilitary dwarves, and the room is sealed with a gemstone window. I sometimes give them an iron sarcophagus.
High-end nobles that die get 5x5 rooms with much more fancy stuff, and of course a crystal window to seal up. Copper or bronze sarcophagii, too.
If I ever get a king and play long enough for him to die, they will find their tomb a large, complex structure with recesses in the walls to house gold and platinum statues. The walls are engraved, the floor is tiled with obsidian and marble, the sarcophagus is gold, platinum or silver, maybe a caged or chained beast or 2 is entombed wit the dwarf. These tombs have secret access tunnels that lead to the original entry point behind one of the statues, which are walled in and made indistinguishable from the surronding smoothed walls.
I would LOVE to be able to either have the tomb specify more about the entombed dwarf, or at least have the option of specifying which engravings are made along the walls. I get tired of Urist RockHammer, my champion dwarf who took down 20 goblins and a dragon before finally succumbing to a slow death by his wounds, entombed ina room with lots of engravings of circles, flies and clouds, with the odd engraving of a dwarf surrounded by dwarves. I would love to have that dwarf's life highlights and death carved into the walls.