I have posted a thread on this matter, how the ranking system will work. Unfortunately, no, it is not the halo 2 ranking system. I will explain, as I had in another thread as well:
To summarize, they are merging the halo reach ranking system and the halo 3 ranking system. How this works, is that they have brought back a variant of halo reach credits. They are different, however. Credits now are not used for ranking up, but only to purchase armor, weapons, equipment, and other things for your custom loadout. Also, they are no longer called credits. They do not yet have an official name for them, but they are temporarily called "Spartan Points".
Now, the halo 3 aspect. They implemented the idea of halo 3's system, by again making it 1-50, with a corresponding symbol from reach, that may be altered. They have also brought back EXP. This is what will rank you up to the next level. EXP is presumably earned how it was in halo 3; winning games. We are unsure how many games you must win for however much EXP, however.
Like I had touched on a little in the above text, your rank will appear as a symbol as well as a number. I am not certain, but I think the symbols may be the same as halo reach. So with that, I will use an example. You have just started playing, and you rank up from Recruit, or level one. You would then rank up to Private, Level two. And so on, until you are an Inheritor, Level Fifty.
I hope this explanation cleared things up for those that were wondering. Thank you for reading!
-Bullet