Not much according to Game radar's lengthy article. Sony's PSN, and Steam for PC's (and even Wii to a some extend) seem to be matching what Xbox Live Gold offers, feature by feature. GR has done their research, calculations and analysis to arrive at the following conclusion(s);
1) Online play with standard features offered elsewhere.
2) Video chat.
3) Mass messaging.
4) Unified Friends list and messenger.
And of those four, Sony’s PSN offers one, two and three for no charge to players. In effect, your subscription pays for... er, nothing more than a list of 100 names you can pull up in any game.
But there is more about how they arrived at it, actually it is a 3 page report that has a in depth report on the matter. Here is the link.