Reading some of the comments here absolutely astound me. It makes me feel like the haters played an entirely different game to the one I did.
Some points:
Ammo Thermal clips ruined the game! -- nope, it keeps the action tense. When you're getting suppressed by a geth volley and your breathing room to attack is only a couple of seconds long, your next shot better be good. Then again, I'm used to fast paced shooters, and I can aim for my opponents' heads most of the time. As for running out of ammo, that only happens if your bullets don't go into your targets. Each chest-high wall-riddled battleground spawns enough baddies for a Widow sniper to clean house without picking up clips, and yes, tagging the heavy ammo crate refills your stock.
No gameplay variety, just duck, pop and shoot -- ME 1 was a cover based shooter, albeit a poorly implemented one. This one does it mostly right. I played my soldier and infiltrator the same way (sniper build) for my third character I decided to go Vanguard, and oh was it a revelation. Jumping from mook to mook, letting off shotgun blasts to the face, I rarely ducked into the chest high walls, choosing instead to go around pillars or take down another opponent while the charge cools down, quickly scanning the battlefield for the next Charge victim. It felt more like the old Doom than anything. I'm starting up with Adept this time around, and even this early on at insanity level, I'm surviving a lot more than I expected, but realized that I had to switch from my standard Miranda/Grunt or Grunt/Legion setup. Now my squadmates are geared to strip away shields and barriers, letting me pluck out foes with Pull, then comboing with Warp, Throw, or my squadmates' Concussive shot. I had to plant singularities in tight corridors with no cover and then methodically beat down enemy defenses until the singularity takes hold. It's slower, but a lot more tactical. I'm thinking of going Sentinel next, I'd like to see if I can go the tank route.
No inventory boo! -- and thank god for that. Seriously, what military organization forces its blackops team to buy weapons personally? That was so immersion-breaking for me, especially when your starting guns in ME1 only sold for like 25 creds while new purchases cost thousands. Now had they made Shepard start out as a freelance merc, maybe buying your own weapons would make sense.
Fewer weapons, no mods -- each weapon feels drastically different from the next. As a sniper, if I want to shoot more often with the penalty of weaker damage, I went with the Viper. If I wanted to clean house with deadly one-shot kills, then it was Mantis/Widow. My Vanguard prefers the Claymore, but I've heard some people stick to the Scimitar or the Eviscerator. If I needed to break specific defense types, that's what ammo powers are for. No need to go meandering through an inventory system, just one button and you're all set. And there's more than one armor. You can mix and match your armor set to either boost powers, shields or health. As for the linear progression of weapon upgrades, it doesn't matter except for the first main act, where hunting for the first three upgrades is necessary to get the sub-upgrades (AP/Headshot). Mid game you'd probably have found the 3 for your main weapon, and that's usually enough.
Linear story -- the progression of one act to the next is linear (intro > horizon > collector ship > IFF > suicide mission), but the missions in each act can be taken in any order. You can even forgo some of the loyalty and recruit missions (the ones after Horizon) if you don't care about some squadmates, so long as you fulfill the requisite trigger. It's about as linear as DA:O. Sure, you can do nearly all the major quests in any order, but completing a set number of major quests or initiating a particular quest pushes the plot forward. Or maybe they mean linear level progression within missions? Because that is true, and aside from the minor annoyance of forgetting to hack that one terminal at the ground floor when you're already at the rooftop of Dantius towers, I'm ok with it.
No surface exploration, mining sucks -- There's the N7 missions if you want to drop down a planet, some of them are very creative. If you really need to roam the vast expanse of an alien world, buy the Overlord DLC. Or just play Just Cause 2. Yeah mining sucks, but at least I don't have to devote more than an hour to it.
IMHO, ME2 is a better game overall. Besides, if I wanted to number crunch, I'd play DA:O instead.