There's a new thread about Eclipse Phase over at rpg.net. Some questions were raised about Eclipse Phase's percentile system, and I thought it'd be good to repeat/summarize my responses over here.

As we've noted elsewhere, Eclipse Phase uses a d100 roll-under mechanic, with some tweaks from traditional percentile systems. To some folks, this may seem an odd choice, and the trend is for new games to have their unique systems, to stand apart from others.

Our intent with Eclipse Phase, though, was to go for "complex setting, simple mechanics." So we opted to go with a d100 system since such mechanics are fast and easy, it's not difficult to gauge odds when the gamemaster needs to fudge numbers, most gamers know the system already, and it's super easy to each. There's a lot of heavy concepts and advanced technology in the setting that can be overwhelming to people unfamiliar with the genre, and we wanted players to be able to focus on that and not sweat the rules.

For people who prefer other systems, d100 is also very easy to convert. This is also one of the reasons we're releasing Eclipse Phase under a Creative
Commons license -- to encourage people to post conversions to other
systems if they choose.

Now, though we play Eclipse Phase up as being a simple system, that shouldn't be
mistaken for saying it's a system that doesn't cover all of the aspects
of the game. The core book is still 2/3 rules text. So it's simple in
the sense that the core mechanics are easy to grasp, the same mechanics
are used throughout the game, and we made efforts to keep things less
complex when possible. We also ditched some rules that are common in
other RPGs but to my mind are pointless bureaucracy, like gear weights
and encumbrance rules. That said, it's a comprehensive system, and
includes rules for everything from switching bodies to nanofabrication
to rep systems to psychosurgery to hacking to mental stress. Combat is
easy to grasp, but the rules also account for a wide range of factors,
from sweeping fire with beam weapons to tactical networks to gravity
effects on range.

While we've used Unknown Armies as an example comparable system a few
times, Eclipse Phase does have some key differences from UA and gets a bit more complex. For example, we have a Margin of Success/Failure mechanic on
some tests, but this is usually broken down as an Excellent Success
(MoS 30+) or Severe Failure (MoF 30+). We went with this because
actually doing the math to see what the MoS is on a test is kind of
annoying, but it's much easier to eyeball whether you got a MoS (or
MoF) that is 30 or more. So when you get an Excellent Success in combat
for example, you do extra damage, reflecting the fact that you got a
nice, solid hit.

There are some things that we leave open to GM interpretation, but
these are usually things that are better handled with roleplaying or GM
decision anyway, such as when exactly a fork diverges enough to become
an NPC, or certain psychosurgery effects, or how long it takes to
travel from Mars to Saturn, etc.

Overall, I think we did a good job of tailoring the rules to fit the game and providing a system that is fast, easy, and consistent. It may not be the system for everyone -- so if down the line you make a conversion to another game system, we'd love to see it. We plan on providing links to such material through our Resources page.