My 425 has been parked outdoors since I bought it in May 2004. It doesn't rain that much in central Texas, nor snow often. Freezing rain every couple of years though.
Things I atribute to being outdoors include keyswitch failure, hour meter failure, rust, discoloration of the paint, seat, steering wheel, hydraulic hoses cracking, light enclosures, and lots of dust. Probably have to lubricate it more often too. When it sits through a good rain the pivot points get rusted and sometimes stick.
After nearly fours years though, it still runs pretty good. I bought it used so now has 6 years on it. I have had to replace 2 wheel motors, head gasket, battery, keyswitch. Not bad considering how rough I have been on it. Cranked it up today first time in 3 weeks. Started right up.
-Tim