This is an almost carbon copy of a different post so Im just gonna add my post to that here with some tweaks to fit this one specifically 
Playable elites - their inclusion means a comparable customization system that requires consistent additions. This means a reduction in the amount of spartan armors created and a reduction of value of the BP given a majority of players use spartans, not elites. This is a logistical omission and one I agree with
Assassinations - this doesn’t really add anything to the game but an additional way to monetize. I am surprised it is not there purely cause of that fact. that being said, it is also most likely logistical. 343 have stated that a vast majority of people turn them off or don’t use them. This would infer that devoting man power to their animation/implementation is just not worth it as a result
Dual-wielding - has a ton of balancing issues that come along with it and really acts more of a gimmick than anything of use
There are 100% things that have come out that are concerning. I think the challenge-based xp is not a great decision, but I am also open to seeing how it plays out. They expanded on how it functions today with there being a daily challenge for match completions of any kind that rerolls near indefinitely (near given there is an end but it’s high enough where I doubt people will reach it in a day). The only thing a challenge-based progression system does not do is make a player feel like they are progressing each match. If there is a challenge that is always there that gives progression per match, I find that acceptable. At the end of the day, all people want it to see a bar fill. It doesn’t matter what the bar is. So if they fill a match complete bar to then get the xp bump, it will work fine. Of course, though, that also means the match requirement cant be too high either. We will need to wait and see.
\The lack of co-op at launch - Honestly, this does not matter to me as I play solo my first couple times through and I would argue to most people that will play the game, will also not matter (I think plenty of people that have no expectations for Infinite will play it which means they lack the baggage of the average Halo fan). Of course its disappointing it is not there but if they could not get it done before launch I am fine with them delaying the feature. What matters is the core campaign is functional, co-op is a plus but not necessary for the initial campaign launch window.
Forge - the lack of forge entirely depends on the multiplayers initial launch suite. If the multiplayer has a bunch of social modes and a decent number of maps, its absence wont be super noticed to those outside of the forge community. Forge is meant to supplement dev stuff and if the dev stuff cant survive 6 months then there are more important issues than forge.
We also dont have an answer to your question. The number of maps and number of modes is unknown but we know there are multiple btb maps as well as arena. What number? No idea. We also know there is all the basic modes (slayer, ctf, oddball, strongholds etc). What additional ones will there be? No idea but ill assume many of the more common social modes such as infection will be there. Out of everything listed, this topic is the most important. If the initial Multiplayer map and mode suite is lacking, that will be the most detrimental failure. Everything is honestly secondary or unimportant