Never heard of this 'Leviathan' DLC before. *googles* Ah, it's RECENT. Came out months after I finished the game, and it extended the backstory without being free! How clever, EA. I know your finances are kind of iffy, but I'm not planning to throw any more money at you any time soon.
Sovereign might well have been more powerful than your average Reaper. They definitely don't all have the same level of power, what with Harbinger being described as the largest and having three main lasers to the usual one.
From what I recall, Sovereign was the equivalent of a Reaper battleship, whereas the Reapers you kill in ME3 (usually through trickery) were usually much smaller, e.g. Reaper destroyers.
The Krogan even use something similar to the Sheridan Maneuver (if all else fails,
sneak up with nukes (also, warp bombs)) on ones landed on the turian homeworld.
They're also not...
self-replicating machines. They can't just sit in deep space and make more of themselves. ME2 and ME3 show us that they harvest (lots and lots of) people to create new Reapers. There's a limited number, and since all the Reapers we saw looked alike (and we know that the Reapers resemble the species they were made) there must be a reason for that - such as that they only survive for one cycle and that there was only one species harvested to make Reapers last cycle, or that the others went to other galaxies, or were still chilling in deep space JUST IN CASE (prepared to adjust their tactics and come back in a few thousand years), or whatever.
Honestly, I was not too disappointed with ME3, but I bought it for XBox 360, waited until after the extended/improved endings came out, and played through ME1 and 2 on the 360 first as well. I also didn't buy any of the DLC for any of the games, so I didn't feel like I wasted a bunch of money on it. It was worth what I did spend, and I wouldn't have wanted to waste my money on the PC version to be forced to install Origin. The only real down-side to the console version is the disk-swapping. But there's no Origin. :V
I felt like they improved the gameplay in ME2, and again in ME3, in that they removed boring mind-numbing shit in each game. They took out the "50% of your time is inventory management" from ME1 in ME2, and simplified planet scanning from ME2 for ME3 so you didn't feel compelled to waste 5 minutes scanning every planet to get all the resources. Sure, the plot kind of went "Well, okay, now you have to beat them somehow, and we can't really just let you come up with ideas and do them because uhhhh this isn't that kind of game and it isn't psychic. You'll follow our railroad plot and like it." But whatever. They're telling a story, which you can change parts of, and choose the ending from among a few choices, but not really rewrite on any grand scale.
How much you do in ME2 basically determines how easy or hard it will be to succeed or get the 'best' ending in ME3. If you did EVERYTHING in ME1, and all your companions' loyalty quests in ME2, and lost nobody on the final mission in ME2, then you won't have to do the multiplayer in ME3 to boost your 'score' (but it can still be fun).
Your actions across all three games, in the end, have little effect on the outcome, and you get the same choices at the end no matter what as long as you have enough 'points' from doing/collecting stuff (or not losing people, etc - basically contributing to the war effort). If you didn't have enough, you'd have less choices. Not sure if it's possible to have no choices and fail. You can do the multiplayer up to a point and it boosts your effective score to twice what it normally is if you play MP enough (it decays over time if you stop playing MP too, but once you pass the point where it checks the effective score for the final result, it doesn't matter anymore). (I didn't need to, but still played it a bit. After that mission, because I didn't realize where the cutoff was.)
However, what happens through the games, the path you took to get there, who lives and who dies, that will still have been different, and EA probably intentionally had everyone get the same choices at the end (instead of the ending being decided by your actions across all three games) to avoid forcing people to replay all three games just to get all the different endings. (There is a bit of customization of the endings, but that is mostly clips)
I assume anyone reading this would have the extended/revised ending DLC, which is basically a free patch as long as you don't buy a pre-owned copy of the game. If you do buy it pre-owned, they want $10 for a key, and you're better off just buying it new if it's $35 or less. In a retail store if you want them to get the least money possible. For a console, so Microsoft or Sony get a cut too (theoretically) and you don't have to deal with Origin.
Have I mentioned I hate that EA used Origin's name for their digital distribution platform, after killing them and desecrating the corpse? Okay, I have a long memory. I'll just go complain about how modern Republicans aren't living up to the model set by Abraham Lincoln now.