Well, I'll post here in the spirit of the OP.
I liked ME1 but I didn't like ME2.
Things that I liked ME1:
1. Character builds- I had to think and plan out how I would build my character and my team. There were many skills to choose from and I found great pleasure in killing my enemies in a multitude of different ways. Biotics were extremely fun to play with; seeing enemies flying all over the place was super fun. I also had the opportunity to have an okay selection of different kinds of armor, weapons and upgrades for both.
2. The team- Shepard's team was diverse yet it was manageable. I felt like I got to personally know every one of them through the conversations. The romance that I played built up very nicely as well. I felt like it was a mutual coming together rather than one side hitting on the other to initiate something. Non-romance characters had interesting dialogue as well.
3. The story- You can play the missions in a variety of different orders. With each mission, you unravel another part of Saren's plan and find out how deep the mess is that you're getting into.
4. The world- The hub worlds for the missions had fluid transitions from one area to
the next; there was always an elevator/doorway or a hallway/path or a
set of stairs that led you to your destination. Most loading screens were disguised as something that was in the world itself.
Interestingly enough, the things that I liked in ME1 were the very things in ME2 that I didn't like:
1. Character builds- I didn't have to put much planning into my build at all, since I had a very limited list of skills to choose from. Ammo upgrades were turned into skills, which made no sense to me. I want to know how having freezing rounds constitutes a skill such as being able to hack an AI or being able to throw something biotically. Shouldn't ammo be relegated to an inventory?
Biotics were made into a pale shade of what they used to be. Instead, a "mage" sort of class has to rely on a weapon during a shared cooldown time between all of their skills. Also, their skills cannot work in some instances, forcing the player to rely on a gun for combat. Yes, I know they made the guns and cover system more like mainstream TPS games. Some folks find this to be an improvement, but I find it to be an annoyance that a mage/rogue class has been turned into a class that has a weapons reliance.
Instead of improving the inventory and keeping it organized, it was done away with entirely. Other than being able to repaint about 2-3 similar looking armors with a shade of color that would look better on a car, the customization options are very minimal.
2. The team- The devs have stated in multiple interviews that there's 30% more dialogue in this game. What isn't mentioned is that there are double the amount of squadmates. Then there's the multitude of cameos, npcs, and clickables that can be factored into the equation as well. Simply put, this does not mean that the squadmates have more dialogue with Shepard in ME2.
Picking up a bunch of squadmates and doing a special quest for all of them seemed like a chore. There were 12 of them and every last one of them had a dire problem, most likely associated with daddy issues. Then when you get to the romance dialogue and turn them down, they completely shut off. Just because I did a quest for the character does not mean that I will feel closer to them. Indeed, quite a few of the character loyalty quests were filled with shooting and a few dialogue choices here and there.
This isn't to say that I didn't like the characters. I liked a few of them, but I didn't feel half as close to them as I did to the ME1 squad. I believe this is because I didn't have the chance to interact with them as much as the ME1 crew.
3. The story- The story of ME2 is based on recruiting a bunch of characters and gaining their trust. It feels like the Collector stuff is a sub-plot. When you learn about the Collectors, you get information in three key points: Freedom's Progress, Horizon, and the Collector Ship. That's out of 22 character missions. There's a major plot imbalance in this game. Instead of discovering what's going on with the Collectors in small chunks, it's dumped on you in a huge mission breakthrough.
ME1 had consistent missions with consistent discoveries throughout. ME2 has 3/25 missions devoted to what is supposed to be the plot. When I find that I don't care too much about the team, it's much less motivating for me to play the game since the game's main thrust is supposed to be the characters.
4. The world- Exploration was cut, as were elevators, walking out of the Normandy's airlock to a docking bay, etc. Loading screens weren't disguised. Instead of riding in an elevator, we get the treat of watching a loading screen with an elevator on it. On top of it all, there is a gigantic "Mission Complete" screen after each mission, rather than having some form of transport pick the crew up or having a summary/debrief meeting with the whole team. Everything seemed choppy and disconnected.
Summary:
With how ME2 turned out and from the way it was marketed, I wouldn't have gotten it if it didn't have the Mass Effect label on it. Everything I loved about the first game was entirely scrapped or turned into a pale imitation of what it once was. Instead of the game that I loved, I find a lot of TPS shooter combat with a multitude of characters that I feel no connection to.
I've lost count of the number of times I've played through ME1; I've gotten every single achievement in it. With ME2, I've played through twice and haven't cared to pick it up again.