Most gamers take 30-60 hours for their first playthrough. It has the nuances of a sandbox, allowing exploration of the galaxy for minerals (with many dead ends).
It also continues the grandiose story and decision tree of ME1, which had a similar setup. While many games are about saving humanity from destruction, Mass Effect 2 actually stands up to the marketing hype of a “rich” backstory.
ME2 gives weight to decisions, from simple, irreversible choices to split-second ethics tests.
The game unfolds (or blooms) instead of sticking to a mechanic of choice for several hours. The game design requires getting to know Shepard’s allies, reacting to their subtle facial expressions, instead of using them as a crutch for backstory.
These intricacies may not even add to the plot, but exist to show internal reflection.
When Shepard is paused for a decision in a conversation, the on-screen character is not frozen, but lifelike in its prompt—swaying slightly in place with human quirks.
These moments show regard for internal life and are artful for that. The level design is also quiet, with periods of silence punctuated with brilliantly animated cutscenes and smooth battle dynamics.
It’s a true American RPG!
This is America’s Final Fantasy! (“ARPG”?) ME2 will spawn many more RPGs that don’t focus on stats, but on pure team composition, skill points, and developing a character’s backstory. I’ll let you speculate for yourself on these values’ roles in American culture.
It’s a masterpiece.
If your question is more of why Mass Effect 2 was 2010’s GotY, look also at its sales numbers (two million in the first week) and average critical response (96 on Metacritic). (Not to take away from Red Dead Redemption.) I’ve listed but a few of the key positives.