Well, to find out what this game is missing, you have to find out what this game had.
1. A great big galactic war. That started too early. Hour one was great. Hour ten was great. By the time you got to hour twenty five, you were just like, "Yeah yeah, wear on.. Yah, I get it.".
Exactly. That's of the few things that always bothered me about ME3's story. Like you said,the first 10-15 hours were great since they served as a buildup,but the second half of the game losses any remaining interesting atmosphere. Especially after Rannoch,there's barely any sense of the bigger picture left (aka. "Ok,why the **** am I here? Shouldn't I be fighting the Reapers or something?"),which culminates in a god-awful final battle.
Most of the Reaper war goes completely behind the scenes,and overall (again first 10-12 hours are excluded,they were obviously meant as a buildup for the rest of the game) the story is extremely lacking. The original script was far better then the actual,released game. Retaking Omega,liberating planets for mercenary groups,going back to the Omega 4 Relay to reclaim a relic for a mercenary groups,visiting the Destiny Ascension,etc. the list goes on. That's something I was hoping for in Mass Effect 3,and that's the biggest thing ME3 is missing;more involvement in the actual war. Instead of saving the entire Hanar civilization by talking to somebody on the Citadel,I wanted to actually go there and solve the problem myself.
Sure,refusing the Salarian's offer or betraying Wrex,or even destroying the Quarians/Geth was fun,but again,there's too much story going on behind the scenes for me to be fully invested in the whole "the galaxy is at war" atmosphere.
Gathering War Assets to boost the war effort is a great idea,but again,it all lacks a general sense of purpose. There are no choices,no gameplay surrounding those groups and ships,no cutscenes,no effects,nothing at all. Only a number you see in the War Room. Plus,none of that **** matters a tiniest bit in the final "battle".
And you can't blame it on the story direction. Heck,the entire freaking galaxy is at an all-out war with the Reapers,how the hell can there be a lack of content if,as Bioware claims,Mass Effect 3 is a completed product -- that is,there was no content or story parts cut due to time and ME3 fulfilled the whole vision developers had?
OT: To answer the OP's question,simply put ME3 lacks content. A lot.
/vent