Diaspora is self-described as a hard science-fiction role-playing game. One of the biggest issues for me with role-playing hard science-fiction in today's world is the trans-humanism element. If you're unfamiliar with this concept, it's basically the improvement of human mental and physical attributes through technology that seems probable based on where modern scientific research is headed. Combined with the tendency of scientific progress to increase exponentially, and it becomes hard for me to identify with what my descendants are likely to be like in just a few centuries.
This contrasts with what hard science-fiction was like when I was younger, where the characters were pretty much identical to modern humans, just transported into other settings.
Diaspora gets around this with the concept of "collapse." Basically, the authors postulate that once technology reaches a certain point it inevitably leads to the collapse of the society that develops it, at least from the point of view of societies less technologically advanced. Whether that's because the people in the society ascend to a higher level of consciousness, or destroy themselves with their folly, is irrelevant. From the viewpoint of the average human the society collapses and a new one grows from its ashes. As a result, humans in the game are basically the same as humans today, without the kinds of changes that could result from certain advances in technology.
In terms of the setting of the game, the development of faster than light travel occurs some time before the collapse, and has led to humanity spreading to the stars at some time in the past. How long ago this first happened is up to the individual game, and is likely unknown. The nature of the way FTL travel works means that it tends to be confined to clusters of a half dozen to a dozen systems. In game terms these clusters are created at the start of a game by all the players involved in a collaborative process that both creates the setting and begins to define what the players expect out of the game.
This process is continued in character generation. The game does not assume that a gamemaster has yet been chosen at this point. All the players are equally involved in creating the cluster, and all of them create characters. Even if the group does know who will be the GM, the GM has no special authority at this point in the process. By the end of the process, whoever does become the GM should have a fairly clear picture of what kind of game the group is looking for based on the decisions everyone made in cluster and character generation.
Mechanically, the game is based on the Fate system, most famously used in Spirit of the Century, and I think the way they've tinkered with the core system is excellent. My favorite bit is how they handle stunts. The biggest problem I have with Fate as implemented in Spirit of the Century is how stunts are handled. They're presented as a catalog of abilities which means it can take a lot of time for a player to browse through and find the ones he wants to take for his character. In a system that's supposed to be designed for pick-up games, this has always seemed to me to present problems.
The designers of Diaspora decided against the catalog method, and instead distilled stunts down into four categories that many of the more detailed stunts seemed to fall into, plus the ability to make up stunts that don't fall into one of those four categories. So, instead of going through a whole catalog, players just have to look at four things, or better yet, decide what they want their character to be able to do, and then with the help of the other players create a stunt that allows them to do it.
Another innovation is the addition of a wealth track to the health and composure tracks. In Fate, damage is handled by the use of tracks. Health for physical damage, and composure for damage taken in social situations. Diaspora adds Wealth to handle the monetary economy of the game. It makes for a nice abstract system that creates economic pressures for the characters without requiring actual bookkeeping by the players. Damage to the Wealth track represents debt. Take enough damage and your character could be taken out of the game just as he could be with health or composure, in this case sent to debtor's prison, or stuck in a dead end job to pay the bills.
I'm not quite as enthusiastic about the four combat mini-games: personal combat, space combat, social combat, and platoon combat. This may just be because I haven't yet had the chance to use them in play. They are all map-based to some extent, and I have a preference for more narrative combat systems. There are also some issues with how range is handled in personal combat that I initially had a hard time wrapping my head around but I think I understand it better now. Of the four, I think space combat is my favorite based just on reading it, possibly because as GM I don't have to come up with maps for each encounter (it uses a simple range band system).
I'm probably most leery about the very mechanistic social combat system, but I think the key there is the advice they give on using it as a tool when the roleplaying is bogging down and isn't going anywhere rather than as a replacement for traditional roleplaying.
I don't want to overstate how I feel about the mini-games. I suspect that my reservations are mostly due to not having had the chance to actually use them in a game.
A brief note on the production quality of the book itself: it's excellent. Currently only available as a hardcover from Lulu.com, the bindery is very well done. The layout is attractive and functional. There is both a table of contents and an index. The contents are black & white, and the artwork is sparse, but this did not detract from the product for me.
I do wish the authors would work through their issues with releasing a pdf of the game, if only because my likely players would appreciate having one available, but they have released pdfs of the charts and forms useful for playing the game, as well as making an SRD available online.
Overall, this is an excellent book, both as a stand-alone game and as a toolkit of ideas for anyone who wants to tinker with the Fate system to create other games.
I hesitate to mention, but feel it is my duty to do so, that the authors are working on a revised edition of the game to be released in both hardcover and softcover versions. I've gotten the impression that this will be released shortly, but I have no insider knowledge as to exactly how soon this might be, especially given that "shortly" is an extremely flexible unit of time in the world of indie RPGs.
Edit:
The corrected version is now on Lulu. You can safely order the book knowing you are getting the most up-to-date version. They're describing it as "corrected" rather than "revised" as it's apparently mostly fixing typos and grammar.