Lol, I hated it....so much. If that's what they took out from the first game, I can see why.
I feel it's just extra fluff with no real substance. Kind of like RPGs with lots of grinding. It adds hours to the gameplay, but doesn't really add anything to the game. If that Hammerhead part was better integrated, focused and less... generic, I may have liked it. As it was, it was so video game-y and felt so disconnected from how the feels, and plays. It detached me from the story and universe, and instead made it a chore... and video game chore. Something that could've been made on the first Playstation.
The underwater part in Leviathan was awesome, and like I mentioned, it was well integrated and focused. That's what made it so good. If it was just one open, generic looking map with generic fetch quests like the Hammerhead sequences... I would've been put off. I was sad that it was such a short part of it.
The underwater part in Leviathan was tight and focused because it was basically a scripted event. You really had no control over where to go. It was pretty much an interactive cut scene. Nothing wrong with that, and I really enjoyed it, but the planet side exploration taps into something different. For people who enjoy it such as myself, it's about exploration and creating a sense of scale. What the Hammerhead quests did was precisely that, making the actual piloting of the vehicle fun unlike the Mako.
By contrast, the levels in ME3 are too focused for my liking, too scripted and too linear, too compartmentalized. While the second game was also this way (but by no means as extreme), there was diversity in activities to make up for it. Several hub worlds instead of one, several different types of gameplay: planetary exploration, playing detective, sneaking into events, dialog games, etc. The few moments of detective work in ME3 (combing the lab for clues, combing the temple for clues, etc.) were nice, but short.
I think the endless waves of enemies are actually more like level grinding than any vehicular exploration. How many times in ME3 do we encounter a moment where device X needs to be repaired or accessed by a squad member while the other two hold back enemy swarms? Similarly, the fetch quests in ME3 were pretty mindless and epitomized "fluff" to me. Like level grinding, it's done with no value aside from upping one's resources. While I realize not everyone is going to enjoy the Hammerhead stuff, my point is that it at least introduced a new gameplay mechanic and contributed to the overall diversity of activities in ME2.
Obviously we have *very* different tastes in gaming, and I'm not trying to win anyone over to my perspective. I'm just pointing out that for me (and fans of ME1), there's a lot of value in simply exploring, reading about the different planets and items, looking up at the sky and seeing the vast ocean of stars and satellites and feeling immersed in a world. Similarly in a game like Skyrim (which you also dislike if memory serves), some people such as myself find value in simply wandering around the world taking in sights and feeling that sense of vastness and immersion. Taking in one's alien surroundings.
What Bioware used to be about in their heyday, and where so much of the value of ME2 lies for me personally, is that it manages to offer a diverse enough array of experiences to cater to a wide variety of tastes. Personally, combat is the least important facet of Mass Effect for me. While I'm not that much of a fan of sequences involving enemy barrages I realize a lot of players are, so I wouldn't call them "fluff." Likewise the detective sequences aren't the most enjoyable for me, but I wouldn't call them fluff for the same reason.