Marck,
1) The panel voices are similiar to the 740's but have additional "live", "sweet", &"cool" voices.
2) I haven't listened to the 620's styles, but most PSR styles are availible from the http://groups.yahoo.com/group/yamaha-psr-styles group. The 2000 has flash memory into which I have been able to load over 30 styles (depends on size). Nevetheless, I do like the internal styles.
3) Not sure about the mic.
4) I carry mine back and forth to church twice a week. Seems as sturdy as the 740. The power supply is external, but is now solid state and light weight.