I would never imagine filming a wedding with less than 2 cameras (I've never tried more than 2, either). One reason is to, for example, capture bride and groom reactions during a particular moment. Another reason, which saved my butt countless times, is to simply have a usable shot in case you need to move your camera somewhere else quickly.
Some other necessities...
Tripods - steady shots are nice shots. A good quality, fluid-head tripod is well worth the money. I'm happy with Manfrotto tripods (501 head, 055 legs...they're pretty good for the money). I also use a monopod which is nice when I'm constantly moving around or I want an overhead shot.
Microphones - a wireless mic on the groom (during the ceremony) or placed on the podium (during the reception) does wonders for anything vocal. A mounted mic for overall, better-quality sound would also be nice
Lights - Receptions, especially, are often pretty dim, so an on-camera light (or a light on a stand, if you can set that up or afford it) will improve your picture quality dramatically. I'm not all about putting things in the way during a wedding, so I use 10w camera lights, specifically the Canon VL-10, because they use the same batteries as the XHA1, and I have lots of them!
Those are what I feel are necessary to produce a good-quality wedding video. Then if you feel like you want to go more advanced, you can look into steadicams, wide angle lenses, dollys......
Hope that helps. I've been doing wedding just over a year now and have learned a lot just by doing weddings. My first was simply with 2 GL2's and 1 decent tripod
and I just worked up from there!