I never played many games of Sins online even though I'm a rather competitively minded player; multiplayer is typically what I end up doing with games. Maybe it would be useful to write down some thoughts - what happened and why.
Sins MP was never (and to some degree still isn't) very accessible. We all know about the hosting problems and worse yet, perceived hosting problems in the form of false error messages. There was also the annoying joined/left channel -spam in the lobby. These kinds of things reek of shabbiness and lack of polish, and made the crucial first impression less positive.
What could be done? Well, the damage from a poor initial experience can't really be repaired. However, the European retail release might be a "second chance" of sorts; it'd probably be a good idea to make sure that MP works as smoothly as possible.
Multiplayer games need to be of practical and predictable length, and many Sins matches don't fall into these categories. The game needs to be short enough to accommodate people with varying timetables from all around the globe. You also have to include the time of getting into a game - this is easily as important as the length of the actual game. A predictable length is another essential point, as it's always useful to know beforehand how much time you need to set aside. Sins could use improvement in all of these areas, though pre-game waiting is not as directly alterable - it's more dependant on a critical mass of players, which in turn is dependant on accessibility.
Solutions? An even faster speed setting, toggle for combat resolution speed (ship health/damage), additional victory conditions, game startup options to affect research costs (faster access to late-game technologies in small and medium games.) Separate toggles for military and civic research costs would be very useful.
The lack of campaign can also be related to lack of accessibility. A good campaign eases players into the game, but in Sins everything is put to the table at once. With extensive tutorials and good campaigns, games can be deeper, more complex and still accessible. Players can deal with complexity when they're guided into it properly. The competitive success of Guild Wars is a good example - at top levels it's an enormously complicated and demanding game, yet it's quite accessible and easy to play for the casual player and indeed has a strong PvE following.
What to do? Obviously, better tutorials and a campaign, maybe for an expansion or sequel. An in-game help system (learning mode in Rise of Nations for example) or computer assisted play (Rise again.) Computer assistance would probably be the easiest to implement, and very useful when coupled with an intelligent help system.
Some posters have stated that Sins has little variety and the tech trees are shallow and largely ignored. It's a fair assessment, and one explanation I remember is that the game had to be streamlined to cut unnecessary complexity. That's nice, but maybe things went too far and a better solution would have been to improve the learning curve. Again, depth is not a bad thing. Kohan, for example, managed to include simple unit customization into a real-time game.
Possible improvements for expansion/sequel? Something to strive for would be "complexity from simplicity", or using simple building blocks to enable more complex plays. For example, the cloaked/detector unit system in Starcraft is simple in nature, but allows for some unexpected plays like physically blocking a ramp or passage with cloaked Dark Templars. Stuff like this doesn't really exist in Sins - the gameplay options are mostly "isolated"; they don't build up on each other to enable new stuff, except for some ability combos. Repulsion is an exception, but it's somewhat problematic since it controls the opponent's movement too much. It's okay to slow down or even deny the opponent from moving, but something is wrong when you get to move his pieces.
Some useful things to add in the future would be more unit interactions (cloaked/detector, minelayer/minesweeper and stuff like that), better hit & run and sabotage units, simple unit customization. The approach to balancing in a good multiplayer game could be summarised by "you get what you put into it", meaning that strategies and tactics that are more difficult to execute should also be more powerful.
Many posters have suggested better modding tools. Indeed, better modability is always a good idea, but in my opinion that should not be a priority for improving online play at this juncture. Modding is pointless if the critical mass of players for online play hasn't been reached, and only the vanilla game can do that. I doubt that the playerbase could currently support a mod, and it's also unlikely that a mod could fix the underlying multiplayer accessibility problems of the game.
Sins is a good singleplayer game, but there's a lot of room for improvement for its multiplayer. I hope that the developers will keep improving that aspect as well. Sins is pretty much the only modern game on the market for some epic multiplayer space fleet action, it's a good niche to fill.