The Science Vessel in my opinion was a very strong caster unit in Brood War and Classic Starcraft. Defense Matrix, EMP, and Irradiate where all strong spells that could easily tip the balance against any opponent. The fact that it was a detector was just icing on the cake.
Moving forward to the Starcraft II Beta, we can see that the Raven like its predecessor, provides the Terran army with a detector and a caster all in the same package. However the abilities that made the Science Vessel much of what is was, have all been discarded ( With the exception of EMP, which now can be used by ghosts.) in favor of some new ones. These abilities are not as nearly as game changing as the ones used by the Science Vessel but are still powerful enough to change a battle.
The first of these is the ability to drop a Auto Turret on the ground for 180 seconds that can attack all visible targets. They are good for warding off a small group of units as well as adding support for your main force. You can often build a few of them as the energy required for them is only 50 which means a full charged Raven can drop about 4 before it runs out of energy. These turrets have an attack much like an upgraded Marine but with more health (150). They can also be used to harass workers.
Its amazing how many of these things you can spawn with just a few Ravens, so you have no excuse not to be annoying your opponent with these.
The next ability is the Point Defense Drone. The Raven spawns these at the selected location for 180 seconds from which it starts to stop incoming missiles by shooting them with a small beam. This is effective against a wide variety of units but keep in mind this does not stop all ranged attacks. Each shot costs 10 energy out of its energy pool (200). This is a very powerful skill that can stop static anti-air defense from attacking air units making drops easier. However these are slightly more expensive to cast then turrets (1oo energy to cast) so make sure you place them where you need them.
Knowing where to place the drone and when to use them will be important so make sure you pay attention to where the missile attackers are during a battle.
The last ability is the Seeker Missile. It is the only direct attack the Raven has and it must be researched. The Seeker missile is slow and must "catch" its target in order to detonate. But when it does, god help whatever is near its target as it does 1oo points of damage to everything in a small area. For 125 energy these can be used to put a dent in a clustered force or to harass workers but keep in mind that the target has to be caught before your missile runs out of time (you get 15 seconds). If you are wanting to use these it is best to have multiple Ravens because of the steep energy cost.
If you want to harass a mineral line, this might be a better way to go then the turret because of its ability to kill workers before your opponent notices. Turrets can be seen at the last minute, giving you opponent enough time to pull his workers away from danger.
As you can see the Raven has a lot to offer, but its biggest role is still that of a detector. It's very important now because you might not always have enough energy at your Orbital Command to do a scan because of the mule being so central to resource gathering. So making sure you a Raven ready might just save you from Roaches or Dark Templar. It is also wise to build more than one (This goes for any detector) less you lose your Raven and are now being hit be hidden enemies.