For me I'd say tyres are the biggest thing. I've had hellish days on snow, sand or mud covered roads and tracks because people brought big trail bikes on road tyres or simply couldn't ride them (and though BMW fitted some sort of trail magnet). I've likewise put semi-knobblies on Triumph Bonneville and taken it to the same places in similar conditions having great fun. There are days and tracks where only a 250cc MX bike gets through, there are others when a Harley Dresser makes it.

My experience is limited to Europe and North Africa (I can never get enough time off work to do the rest ), but i'd really say it depends how you ride. Which is worst for you, a day at less than walking pace in the mud (never had one myself), or doing 25 mph on the motorway while the trucks howl past in a cross wind (the XT was down to 35 in places)? Can you live without a spare sleeping bag or do you need the 60-80 hp to lug your gear? How tall are you? How skilled off road? How upset do you get if you have to take a 100 mile detour because it decides to rain?

Trail bikes tend to have fewer things that drop off or get ripped off when they meet the mud. My BMW F650 and Yam XT had nothing modified except tyres. My Bonneville has a new set of silencers about every two years off e-bay because I ding 'em and they rust faster than normal. For a bigger trip with more than a few miles off the tarmac I'd need to do a proper exhaust mod to prep the bike. It might well be easier or cheaper to just go buy another F650.

At the risk of starting a fight (and please remember I'm 5 foot 7/1.7m and weight 75 kg) I'd say a BMW R80GS or ST or F650 was about right for how I ride (The Bonneville with knobblies is similar) while XT's are heading towards too much trail bias and KTM's are just too much. That's only for me though.

Andy