Most RPGs (role-playing games) fall into two categories: linear or open world. Neither is better than the other. Each has its advantages and disadvantages, and whichever you prefer is up to you. The upside of linear games is that they are tightly focused on the story and characters, and their environments are generally more fully realized and detailed, as the creators don't have to worry about creating huge, expansive worlds. The downside is that they force the player into a situation where they have no escape. If you don't like what's going on, you don't have the option to go to another part of the game world and do something else for a change. Side missions are limited, if they exist at all, and your ability to maneuver or explore is minimal.
As for open world, the upside is that you have an entire game world at your fingertips. You can go off and do whatever you want as it pleases you. The main quest can be put on hold so that you can go explore random caves, go fishing, or simply choose a direction and see where it takes you. The downside is that these games can encounter a problem with lack of focus; the sheer plethora of choices at your fingertips can paralyze you into inaction. You might lose interest in the story because you aren't being prompted or reminded of it. And the characters might be simplistic caricatures, as the game creators have to make a lot more of them to populate an entire world.
Mass Effect 2 promises a great deal. Not content with a world, we are given entire star systems to explore. We can cheerfully plot the Normandy to wherever we please, provided we have the fuel to back it up, and the amount of systems you can travel to are many. Each system has a handful of planets that can have paragraphs of description to them, adding further to the idea that you are a tiny part of a gigantic universe. The game itself takes you all over, from the dank, Blade Runner-esque urban ghetto of Omega to the clean and polite United Nations-like Citadel Station. You can land on planets in perpetual blizzard as well as those that contain sparkling waterfalls and astonishingly vibrant greenery.
The Codex (in-game encyclopedia) is incredibly effective at conveying this depth. Through it, we can get the deepest details of any given species: their biology, mating habits, military tactics, and diplomatic approaches. We can read of past historical events, wars, treaties, and agreements. We can read the hard scientific concepts behind the ships that populate the game, their interstellar travel systems, and their weaponry and armor. The characters that come to populate your squad provide further glimpses of an immensity beneath the surface, quirky personalities providing stories that make you want to see and visit the places that they describe.
But, while Mass Effect 2 talks the talk of an open world, the universe it gives us is woefully and disappointingly limited. There are probably a hundred or more planets that you can visit in the game, but you can actually land and explore only a handful. The vast majority merely give you the image of the planet, a little blurb describing what it is, and allowing you to scan and gather resources from its orbit in a swiftly tedious mini-game. The galaxies and worlds described in the Codex are barely touched, creating the frustrating feeling that there should have been so much more to the game that just isn't there.
What's more is the fact that, of the worlds you can visit, the amount of space you have to explore upon them is typically very contained. Most combat missions only truly take place among a handful of rooms. Having only a few small areas on an entire planet feels like a cop-out. For example, during your time on the entire space station of Omega, the only area you get to explore is a single nightclub and a slum. On Citadel Station, all you get is a strip mall and a solitary room for 'the Presidium' (supposed to be Mass Effect's entire Galactic Senate). This would be alleviated if the gameplay were spectacular, but it is only a generic cover shooter plus force powers. This would also be countered if the areas were densely populated with interesting figures to interact with, but the majority of NPCs (non-player characters) are just visual fluff that you cannot interact with.
This criticism is not to say that Mass Effect 2 is a bad game by any means, but is merely to highlight that it is a game that promises and makes you want to believe so badly that you have an entire universe at your fingertips, and then gives you little with which to satisfy that dream. Though it seems such an enormous game, it is merely a pretender, likely subservient to the belief that people who play it will not have the patience or interest in exploring. Thus Mass Effect 2 will likely have a bittersweet finish for me, an enjoyable experience tempered by a lament of what could have been.