Here is a simple break down.
Final Raid Bosses with blizzard generally drop an iLevel tier higher than the normal bosses in the dungeon as a bonus for clearing it and considering its one boss its a bonus.
As for Naxx 25 and OS 25.
Naxx 25 is the simple fact that Kel'Thuzad is the last boss, hence following the tier higher loot system.
OS 25, you only awarded this by doing the hard mode which effectively brings the fight up another tier.
Maly is right up iLevel 213 on 10man and 226 on 25man
Now for Naxx which is iLevel 200 on 10man and 213 on heroic.
OS 10man, is just like naxx but has Hard-Mode Modifiers.
iLevel 200 for Normal Mode (213 for Hard-Mode) no Drakes
A bonus iLevel 200 (213 Hard-Mode) item for doing it with 1 drake up.
With 2 drakes you get 1 bonus iLevel 200 (213 Hard-Mode) item and a bonus iLevel 213 (226 Hard-Mode) item.
Once you see the pattern, you can catch on quickly.
As for Ulduar, blizzard stated they would include more Hard-Mode Bosses.
I'm expecting Freya, Hodir, Mimir, Tyr and Thorim to have Hard-Modes there.
Engaging them on Hard-Mode they should drop iLevel 239 items on 25man and iLevel 226 on Normal.
There is a chance they could introduce a new tier of Emblem with Ulduar also.
Simple breakdown:
iLevel 200
Naxx (Normal) [7]
OS (Normal) [7]
iLevel 213
Naxx KT (Normal) [7]
OS 2 Drakes (Normal) [7]
Naxx (Heroic) [7]
OS (Heroic) [7]
Malygos (Normal) [8]
Ulduar (Normal) [8]
iLevel 226
Naxx KT (Heroic) [7]
OS 2 Drakes (Heroic) [7]
Malygos (Heroic) [8]
Ulduar Hard-Mode Bosses (Normal) [8]
Ulduar (Heroic) [8]
iLevel 239
Ulduar Hard-Mode Bosses (Heroic) [8]