rooting ur phone is at ur own risk so make sure u read a lot about rooting in the net first and understand it otherwise u'll regret it...
rooting gives u the admin control of ur phone so u could do anything that unrooted phone cannot... example is the superuser access which will granted access to install root apps like app2sd, move2sd enabler, titanium backup and many more...

read the rest here...
http://forum.xda-developers.com/showthread.php?t=961858
nothing big actually only that if anything happens u could just do the hard reset without losing any files or data like phonebook or sms's or anything that u put in manually...
if u root using soc 1.7 there is a button for unroot there so click that button first before sending for warranty...

nope not all can be move to sdcard unless u use move2sd enabler and gave the permission to do so... but it wont move any apps that needs to run on android services or need to use any sync service... and also apps that have widget