One of the biggest problems with Sins is the lack of players online. For something which sold hundreds of thousands of copies (over 500,000 by Fall 2008) and which won IGN's "Best PC Game of the Year", there's something seriously wrong if one can usually only find 20-50 people online, even during the peak hours in the evening. I can think of a few contributing factors to this:
There've been numerous serious bugs and compatibility issues, especially in the past, from what I've heard. For instance: the Mesh error is very common for newbies just starting to attempt to play online, which is relatively easy to fix if you know what you're doing, but can prevent newbies from ever being able to play online in the first place - this may well have prevented thousands of prospective players from checking out the multiplayer scene. Another serious bug is minidumps, which were apparently extremely common in the past while playing a game, but still appear frequently enough to be very discouraging. Also, quite a few players may have been interested in multiplayer and clicked the big "Multiplayer" button on the main menu, only to find that it leads nowhere unless you already know what you're doing and have a LAN game set up. I think the labeling of the "Multiplayer" button was a serious design flaw. The learning curve for multiplayer is also a huge problem: the game may seem deceptively simple at first (complicated but not overwhelming) when in reality there are hundreds of things to learn to become a good player, none of which are covered in the horrible tutorials. Odds are, someone coming online for the first time will be completely annihilated by the regulars in at least the first 30 or so games they play, before they learn the initial ropes. For many newbies, this may be discouraging enough for them to simply quit competitive ICO permanently and go back to killing AIs in single-player or comp stomps. This is a self-perpetuating system: low player counts populated mostly by experts makes it extremely difficult for a newbie to willfully trudge through the 50+ hours necessary of playtime (while being killed) to become halfway decent. There are too many experts and not enough noobs or semi-skilled players online for genuine newbies to have fun playing the game against other humans... which results in low player counts because 95% of the newbies aren't going to stay.
If most of the above issues aren't fixed in Rebellion, I fear we'll be seeing very low player counts there as well, once it's released. Some of the errors are likely much more complicated to fix (minidumps), but the ridiculously common Mesh error at least should have been extremely simple for the developers to fix.
I hope that Ironclad fixes the bugs and then makes getting onto multiplayer in the first place much easier. For instance, perhaps occasionally have pop-ups on the main menu saying something like "Player X is looking for players for a 2v2 game online! Join now?" Or something like that. Attracting newbies to at least experiment with multiplayer, hopefully against other not-so-skilled players, would be one of the greatest ways to improve the multiplayer scene for Sins, in my opinion. One idea is to allow newbies to have handicaps, like 2x income or something like that, to be decided by the game host - advantages like the harder AIs have. This would give even the better players on ICO a challenge against newbies (depending on the advantages), and both players would have a chance of winning. Still, hopefully Rebellion will do well enough for enough newbies to come online so they can play each other, instead of being annihilated by the experts and then discouraged from ever logging on again.
Just a few thoughts I hope the devs will seriously consider. A healthy, active multiplayer community is one of the greatest keys to a game's success, I think.