I'd only played ME3 after I'd already heard that everyone felt the ending sucked. And when I played it... yeah, I could see where they were comming from. It wasn't good. After the fun and inovative handling of multiple endings in ME2, getting a poor man's version of the Deus Ex ending (the first one, mind you) was a disappointment. And Deus Ex managed it without gaping plotholes, a complete tonal shift, extremly forced drama and had some fun gameplay in the choice.
To elaborate on those points, well, I don't think I need to cover the plot holes, do I? The tonal shift is also big. Suddenly the big, overarching conclusion is that organics and synthetics cannot coexist? What the hell did we play ME2 and ME3 for then?
By forced drama I mean how the negative side effects of the choices seemed shoo-horned in. We established in ME2 that the Reapers aren't just ordinary synthetics, so why is there no way to destroy them without taking out all synthetics along with it? Plus, all choices end with Shepard dying, but for completely different reasons. Destroy the Reapers? Your synthetic parts will be destroyed and kill you (what, will all toasters cease functioning too?). Control the reapers? Ohh, too bad, we need to upload you consciousness in a way that'll kill you. Synthesis? Yeah, that needs to kill you too for some bizare reason. It's all so convoluted. If the Crucible had gotten damaged in the battle for Earth, so that any power surge would have resulted in the thing blowing up or something, it would have made more sense. Now the creators of the Star Child seem even stupider. Not only did they try to solve the problem of races being wiped out by their synthetic creations by building a synthetic creation and giving it complete control, but everything is designed to kill the user if you want to change anything.
And by gameplay in the choice, I refer to something the new Deus Ex screwed up as well. In that game as well as ME3, the choices are made with a single push of the button at the last second of the game. Okay, you need to walk a bit in ME3, but there's no gameplay involved. Quicksave, and play all endings one after the other. The first Deus Ex meanwhile made you work for the choice you made. Several character you meet tell you what they want you to do as you approach the last two levels and what it will result in. And once you've chosen, you need to travel through the last two levels completing different objectives. It makes the choice feel more organic, instead of the stupid idea of the villain's lair having a single machine with 4 buttons that lets you do exactly what you need to stop him.
The EC was better, but still ultimately dissapointing. Of special note is its attempt to fix some of the worst plot holes in the ending. But it did so badly. For example, the plot hole of how your companions end up on the Normandy. The EC, which was supposed to 'clarify' the original ending, showed the Normandy land and pick up your teammates right in front of you. Yeah, that's not a clarification, that's a retcon.
And it's a bad retcon, because it just introduces more plot holes. In no particular order: Why don't we just medigel that shit? The pickup scene only shows your two teammates of the last fight being picked up, but all your crew members were at the rallypoint with Anderson, so when did they get back? How does the Normandy get there so quickly from orbit? Why doesn't Harbringer blow up the easily recognizable ship of his Nemesis since ME2 when it's parked right in front of his deathrays for a full minute? If the Normandy can just fly down there so quickly, why doesn't it do so again after Harbringer leaves, letting those two useless marines quickly enter the teleport beam?
In other words, they didn't put much thought into this. It smells of desperation to give some justification to their original plothole-filled ending, so they just came up with a bad excuse as to why it totally makes sense for your teammates to be up there. We also get an extra scene telling Joker to retreat, because that's also something Angry Joe complained about. But why is he ordered to retreat? Shouldn't the fleet keep guard of the Crucible until they're done? And why is the Normandy the only ship in a Mass Relay jump, if everyone was retreating?
In short, EC or original, the ending is a dissapointment, not nearly up to the standards of the writing before this moment, even in ME3 itself. The sage deserved better.