It was fantastic. Ending was nowhere near as poor as all the hate seemed to imply, I was almost disappointed at how mediocre the ending was as opposed to terrible since all the hater's hyped up as to how abhorrent it was. Then again I might just be thick skinned, since as a horror buff I'm surrounded by nothing but poor endings to films that never deliver. I'm assuming the majority of the hate was directed towards the DAY 1 DLC though which is justified, Javik is a great character but the DLC mission was poor, so one would be conflicted as to whether it's worth $10.

I struggled on my Insanity Vanguard run at first, then I broke the game once I got a light shotgun and got my cooldowns to 190%. Biotic charging everywhere every 2-3 seconds.

How does one unlock new guns in co-op? I've gotten weapon mods and new characters but never a new gun.

Buying weapon packs, veteran ones offer "uncommon" weapons/characters whereas the spectre pack offes the rarest ones. That's the way to get more advanced weapons and upgrade them too, ie I don't think it's possible to get Widow sniper rifle from recruit pack and get upgrades for it from them either.

I thought it got a lot easier when I got the drain ability for shields with my vanguard on insanity. Or more fun, Since it was less tedious to take enemies shields down I'm not sure. (I use my favorite characters (ash and liara) instead of the most effective combo  instead of the most optimal ones.

I used a weird strategy most the time, with a solarian pistol gun with upgrades with the projectiles that sit on the enemy and explode, and a  submachine gun. And no shotgun (I generally just don't use them in shooting games for some reason :P), I probably played it weird or wrong haha. 

There's no wrong way to play, just the way that's most fun to you. I just love how flexible the combat is in ME3. I just wish you had more than 3 active skills to use in Co-op though. Should have been 4 active skills and 1 passive skill I reckon.

Did anyone notice that after finishing Priority: Rannoch, the Minos Wasteland system keeps flashing and the planet Aequitas has it's name green tagged (as if a mission was supposed to happen there), but there's no actual mission for that planet?

That didn't happen to me. In fact after Rannoch 2 active quests stopped flashing and just disappeared (one of them I had just picked up). Needless to say the game is pretty glitchy.

Didn't notice anything about quests dissapearing, but that's mostly because I try to do every quest before doing the priority missions. Were those two quests on Rannoch itself? Because if you do Priority: Rannoch before doing those two (the geth fighters and the geth server), they become unavailable.

I've been reading about this glitch I found over at the Bioware troubleshoot forum section, and it seems it affects PC users mostly. 

One of them was a Rannoch quest but the other one I got when I went to the citidel after Rannoch and it shows up in my Journal but not in the galaxy map. The quest is from an Elcor Ambassador. 

That means you most likely have the item. Just talk to him again. 

A quest from an Elcor ambassador? I didn't find any like that, the only quest I got from an Elcor (outside of Purgatory) was to find the Code of the Ancients by scanning their home planet Dekunnia.

Edit - Nevermind, I found that Elcor ambassador (the option to talk to him only appeared after Priority: Thessia though), it gave me the Dekunna: Elcor Warriors side mission, which is only a scan mission (already had scanned Dekunna and it's moon), so I just had to talk to it again. The rewards are only exp and rep though.