Minecraft wiki entry for Explosion, associated text: Explosions with a power greater than 100 look mostly the same from the outside, as only certain lines are used to determine if a block breaks. However, some of those lines continue underground.
Explosions that occur in flowing water or lava apply propulsion to entities, but don't affect any blocks, regardless of the blocks' blast resistance.
Underwater explosions don't emit smoke particles.
Explosions can redirect projectiles, including ender pearls.
An explosion just after a mob's death can propel the mob's body.
If primed TNT explodes in a large, solid cube of stone blocks, it creates an exact 3×3×3 cube inside. Experimentation confirms that a TNT detonation causes a 3×3 hole in a solid block of anything with a blast resistance less than that of water, but more than 2.5 (e.g. crafting tables). This implies that 3×3 is the minimum possible result of a TNT detonation without the blast being resisted altogether.
If a falling sand entity falls into primed TNT when in water, it deals block damage.
Every block that has a higher blast resistance than cobwebs can block a ghast's fireball and remain intact.