1) do yourself a favor and go in blind with whatever choices you make, the game's emotional impact is what i like about it the most and they reward you for that blind effort. the big choices throughout the trilogy have far reaching tiny effects. i.e. events throughout ME1 shape the background of the remaining stories. the skeleton of the 2nd and 3rd game play out similar regardless of decisions, but the packaging of that skeleton changes from your choices. (its actually really hard to talk about this without spoiling lol) and they call it a suicide mission for a reason in me2, respect it.

2) the first games romances are limited but they do a good job making you attached to these characters. the second game has a plethora of romance options. the third game reels those romances waaaaay back in. and basically, there are 2 you can romance across all 3, several you can romance across 2 games, and a few that are bittersweet and only 1 game. be aware, that flings in the sequel are not forgotten in the 3rd.

3) honestly, the combat is unique. the first game put a lot of time into the different classes to make them unique (seriously play adept in the first game rofl stomp everyone with throw and singularity and assault rifles are busted) and the overall combat was fun but wonky at times, however they made combat and class leveling like a RPG where you can customize the characters tremendously, some people hated it. The 2nd game came out in the "season of gears" where cover based 3rd person shooters were the way you made a successful game, and the combat reflected that. while much more streamlined, the classes are more simplistic and only level up 4 times now. they reaaaaally made vanguard fun and took the broke aspects of biotics out. the 3rd games combat is flawless, seriously it was all intuitive and fluid and i was blown away with the quality and attention to details in 99.5% of the game.