In the first Borderlands game, the Siren was a phase shifting Demon who could cause massive amounts of elemental damage to enemies. This time around, you get a bit of the same but with much more emphasis on teamwork.
In Borderlands 2 the Siren now has the ability to phase lock enemies which in a nutshell means they are temporarily lifted into the air and rendered immobile allowing you and your friends to concentrate fire and take them down. The Phase Lock ability also has an assortment of elemental abilities for those more accustomed to the play style of the previous game.
What should never be ignored by any Siren who wants to play on a team is her ability to revive downed teammates with her phase lock ability. Coupling this ability with skill reductions to her cool down makes the Siren a formidable force that can take the heat off of fellow vault hunters and then turn around and revive a downed teammate surrounded by enemies through the scope of a sniper rifle.
Additionally, the Siren also has abilities that restore health in various manners. What makes the Siren stand out the most is how frequently she can use her abilities. In most cases, while playing, you can use your ability, kill a few Psychos, and wait for it to cool down before your teammate bleeds out across the field.
For those who are interested and not insecure about their masculinity, I strongly suggest you try the Siren. You might even find yourself as the team's new cornerstone.