I liked that AC1's main character didn't have any personality to slow the story down or complicate anything. It's short and simple, which is something AC2 is not. AC2 feels like it's going to go on for a long time, and they actually try to establish the characters and make them meaningful towards things, but I find it failing in two ways: 1. Everyone's names are so incredibly Italian, and there are a lot of characters that are all very hard to remember/say/associate with any one person. 2. They all feel half-developed. Like they introduce all sorts of colourful people who either never really get seen again, die, fade away into the forgettable support cast, or are only ever MENTIONED after they are introduced. The only people I really know are Ezio, Da Vinci and Mario (who seems pretty absent right now, but he's very memorable).
I hate the Altair snappy selfish behaviour. I played a few more ours and the decided it was not for me. Liberation happened to be a very spinoff.
As for Mass Effect… well It sounds very RPG-like, and I am not one to warm up to RPG-type games. I'd rather get pulled into a universe and get to experience a journey taken by our main character, than have to mould my own at a few key moments. This is why games like Minecraft, Skyrim and Fallout do not interest me. Because I have to actually look for a cause to take part in, and to be honest I am too lazy to look.
Maybe there would have been a time a game like Mass Effect would appeal to me, and maybe there will be again someday soon. But as it stands right now, I'd rather sit back and follow the flow of the games I play.
Mass Effect 1 is a true RPG. Mass Effect 2 and 3 are more action oriented. Out of all 3 Mass Effect 2 is probably the most action packed. I'd recommend you to pick up ME2. If you like it ME3. Then you can always buy the Trilogy pak. ME1 is only available in that Trilogy pak… which happens to be rare and expensive. It was the first time ME1 came to PS3 and well worth your money when you’re a fan. I got an NTSC-J version. Much cheaper than t PAL version and there’s no difference really. Unless you want to buy DLCs, but PS3 master race. ME2 has almost all DLCs on the disc, because it released late.
Mass Effect is split in 3 sections. I'd call them passive and active.
Passive:
Conversations through dialogue wheel.
The ability to ask questions, talk with people, solve conflicts, give advice and possibly romance on of your crew. The dialogue wheel is also part of interactive cutscenes. There are very very few cutscenes that are not interactive. So technically, you're always playing. It’s a lot of fun to see your character come to live and talk to your crew. They are the most realistic characters you will encounter and you will bond with them over time. None of these characters are forgettable and prepare yourself for a lot of ‘feels’. Like I said, a second life.

Exploration
There are huge facilities and cities that you can explore. Find more interesting people, side quests, maybe grap a few drinks and dance? You will get to see everything. Walking to the hospital, take the elevator and back to your ship. And you know what? It never becomes a chore. No matter what you're doing, the game makes sure you're having fun. Speaking of fun, the game adjusts itself. When you don't import a save file from an earlier game, but will make sure you're up to date anyway. The game thinks for itself.

Active:
Aside from being a great RPG, Mass Effect is a 3rd person shooter that puts many of its genre to shame. You have your shield and your actual health. You can take a gun of each class. 2 handguns, an assault rifle, shotgun, sniper and a heavy weapon. Aside from that you've got an arsenal of abilities that fit your class. You will be able to choose a class in the beginning of each game. It's cover based like Uncharted, but much smoother. Especially ME3 where you dodge to each side. You can also issue your party members to do special tasks. Hold position, attack certain enemy etc.

But you can also do this:
Upgrading Abilites
Becoming stonger, equick different weapons and armor. If you think it's boring, you can chose to auto level up and the game will upgrade everything for you. And not randomly, it thinks remember!

That's as short and spoiler free as I can be 
Beautiful CG Trailer with Two Steps form Hell
Represents the feel of the game.
And what the hey. Give the dog a bone, here's some gameplay:
http://youtu.be/8kYNqTs2WEQ?t=2m17s
*it's one of the first missions. Be kind.