I've read the online reviews of the game, and it looked like a very very good game. I also really like the fact that I can just buy it from the site, but I decided to download the demo first just in case.
And boy am I glad I did...
Because to tell you the truth, I'm disappointed. 
Now I'm making this thread for several reasons - I want to say what I didn't like so if:
1) It's just because I missed some feature or because the Demo is so short, and I'm wrong and the game rocks, then I want to know about it.
2) Heck, maybe someone will someday implement these comments and make a game that I'll really really really like.
So without further ado I present my complaints:
Generally, it seems to me that the game took the depth of RTS and added the pace of 4X. And IMO what should've been done is taking the depth of 4X with the pace of RTS...
1) The playing field, while it can LOOK huge, is actually very small. There isn't any room to maneuver in a gravity well, and there isn't anything at all outside gravity wells. So instead of a huge map, what you really get is lots of "dots" of engagement with phase lanes between them, like little miniscule maps interconnected. The distances inside the gravity wells are small, and ships will always encounter each other there. Oh and to boot, in the openness of space... the game is on a 2D board...
2) The former point connects to the current one, which is battles. My ships just sit in place and fire of the enemy, which also sit in place. No maneuvering, no tactics, no strategy. It's the sitting ducks game. I won't get anything from sending my ships around the planet, or from brining more ships from another direction. Pincer move and flanking don't seem to exist.
Mind you, I'm not talking about micromanaging battles here. I'm talking about flanking a foe by bringing a fleet from another planet.
Really, 10 years after HomeWorld I'd expect more from real time space battles.
3) Still on battles - the differences between ships, if they have any real impact, are not clear to me at all. I realize this is a point that can be especially influenced by the limited nature of the demo, but I really don't see why or when I should build one ship and not the other, especially if the other seems to have more firepower.
Seems that battles here are only about who have more and bigger ships, and there is no point in building the lesser kind when you researched the bigger level. And when you have a big enough fleet you just throw it straight at the enemy, and let it fight. No other consideration necessary.
4) The game environment is very simplistic. Only 5 planet types with gas being useless, all the same size, no moons, nebulas and such have no real impact on the game and are treated as planets. There isn't much exploring to do other then finding out what kind of "planet" is in the end of the next phase lane.
5) There is no real diplomacy in the game, or so it seems. Gone is the complex diplomacy of GalCiv2, we are back to simple RTS diplomacy. From a game that presents itself as RT4X, I really expected more.
6) Planet development, and in fact any development is very simplistic, even shallow. 1.2.3.4 Habitats and we're done. Slap another shelter on, etc etc. I don't need a real development plan for my planets. Same goes in space around them. Just place whatever structure wherever you want, it's all pretty much the same anyway.
7) The research tree is very RTS in nature. True, it's not the simple and small Starcraft or C&C tree, but it's no bigger then Rise of Nations or Earth2*** (Choose the year, all the games apply). This is not the extensive and long 4X research that forces you to choose a direction and game-long development strategy (and rewards you with being better then others in your areas of expertise).
Well, I think these general complaints pretty much covered the whole game... which is sad.
I can go on and on with detail about how I would make a game, but after I just bashed Sins I doubt you'll read it. 
I will point out some games and parts that did it right though:
Hegemonia - now that's a game that did A LOT of it right. The English sucked (really, it did) and there was NO point in building smaller ships once you researched bigger ones, but in other areas it REALLY shown. Solar systems were HUGE with plenty of open space and planets. Connected through wormholes to other systems the effect was very similar to Sin's phase lanes. Only here the gravity well was an entire HUGE solar system. I expected Sins to take the good here and improve it.
Hegemonia also had a relatively long and complex tech tree, and a good heroes system.
HomeWorld - I'm sure you saw it coming, but it's for a good reason. HW used all the 3D of the 3D map, had amazing space battles with maneuvers and diversity. Players always had many ship types in their fleet, etc etc.
GalCiv2 and MOO2 - Diplomacy, research trees, 4X-iness... And considering that Sins was made by those who made GalCiv2... tsk tsk.
Well anyway. I'd expect a game that calls itself the first RT4X to combine the best elements of these games and expend them. I really feel that there was a great miss here.
P.S - As I said before, I only played the demo. If I'm wrong with my observations then please correct me. I'd like to play a game that is what this game PRESENTED itself to be. I just feel that what this game is in reality is not even close...
P.P.S - I read some of the suggestions for expansion, and I must say that lots of those suggestions seem like things that should've been in the game already. Feels like Sins is an incomplete game, It got the skeleton (and parts of it I don't like as you've read) but lots of the meat is not there, and just maybe it'll appear in the expansions (or mods).