My son, who knows all too well of my affections for things Traveller, bought me a copy of QuikLink Interactive's "Traveller T20", which is a D20 Modern take on Traveller. While T20 itself is out of production due to an expired Traveller license, the game engine lives on in SciFi20.
Others have reviewed the game in detail, so I won't review the product. Instead, I'm going to point out some things that interested me in the first partial read through.
I'm familiar with D20, especially in its 3.5/Pathfinder incarnation, though I'm not what you would call a professional D20 player, so a lot of power building special interactions are probably lost on me.
Having said that, I think they way T20 handles certain skills and feats is very clever. The best example is Piloting, which is a skill. You can rank up your piloting skill as a general stat, but in order to actually pilot something, you need to take a Feat, such as "Spacecraft" or "Aircraft". So you can have your insanely great spaceship pilot who has no idea how to fly a helicopter, but can pick it up pretty easily (on a level up) and still be a great pilot. This meshes nicely with Base Attack Bonus and the various Weapons feats.
Combat is pretty standard D20 (d20+bonus against AC, roll damage if you hit) but T20 separates your "hit points" into "Stamina" (which are equal to D20 hitpoints) and "Lifeblood" (which is your Constitution score). Any time you take damage, it comes off your Stamina, and at zero Stamina, you're out cold. In addition, any damage you take comes off your Lifeblood, but only AFTER armor has had a chance to stop it. Armor is very important in T20. It not only increases your AC, but it reduces damage to Lifeblood, keeping you alive longer. The way the AR rules work is a bit convoluted, but probably not too hard to handle in practice - each point of AR removes the lowest rolled dice of damage until there is only one dice left. Then, any remaining AR removes points of damage. So a 2d10 attack (3, 8) against an AR 5 armor would first cancel the 3 with one point of AR, then use the remaining 4 AR to reduce the 8 to a 4. So in the end, you'd take 11 Stamina and 4 Lifeblood from that hit. Two other points: nonlethal damage just comes off of stamina, and once you're out cold, additional damage goes straight to Lifeblood, bypassing any armor.
I also like the approach they've taken to building space ships. The math works out to be very much like Classic Traveller in the end, but the approach is totally different. Each hull size requires a number of "units" to power it. Say for example, a 100 ton ship needs two power plant units installed, two maneuver drive modules, etc. Each module is rated for performance. So if you want 4G acceleration, you'd need to buy two 4G gravitic maneuver modules, and a powerful enough power plant to run them. This opens up a lot of RP chrome. "That last hit took out your #3 maneuver drive." instead of "Your m-drive just dropped from a D to a C rating," which is how it would have worked under Classic Traveller.
In addition, you now can choose your avionics, sensors and computer separately, and they all require a certain level of power and control rating (provided by the power plant and computer, respectively). Again, more detail than CT, and in a way that would help bring the setting to life during play.
Character creation has an interesting concept. Traveller characters do not classically start of as 18 year old newbies, they are for the most part ex-service personnel who are already skilled. But D20 games have levels and XP. How do you reconcile those two things? Well, T20 allows you to have a prior career in which you gain a somewhat random amount of XP which you can use to level up your character. The skills and feats available to you depend on the career you have chosen, and it's possible at higher levels that you will not get enough XP to level up during a term.
Another character building rule T20 has added is the idea that your homeworld's statistics grant you certain skills and feats. CT does not care about your homeworld, and Mongoose Traveller handles things with their "level 0 skill" system, which I don't really care for, since it implies that all skills are trained-only skills. D20 in general and T20 in specific of course allow untrained use of certain skills for anyone.
Any way, since pretty much everyone I play RPGs with is familiar with D20 rules to some extent, I may wind up switching to T20, even though CT still holds a special place in my heart. Fortunately, T20 went through some trouble to remain compatible with CT adventures and such, so it doesn't have to be just one or the other.
That's enough rambling for now.