Welcome, Kungfu...
1) I haven't been abroad for that long, but I tell ya, carrying a huge pack (my Jansport Rockies is immense) can be a bit tiring, but you also have the peace of mind that you have room for all the shit you need, and then some for all the knick-knacks you pick up along the way. But 3 months is a looong time, and it's more likely that the further you go along, the more stuff you'll send home via DHL or UPS or whatnot.
2) On my last Europe trip, on the way back from Barcelona to Amsterdam, my bag topped out at 27 kilos, 2k over the airlines' limit, but they let it slide. It seems the heaviest items I tend to carry everywhere are bottles of booze... But even before I went over w/ all the drinkable souvenirs I bought, I felt like I had a pretty heavy load whenever I left one city for another.
2) --following your numbering system-- You might have some issues squeezing on to some buses or local metros, but longhaul transport (i.e. coaches, trains & planes) should pose no real problem. 'Course, I've gone on backcountry trips with 60-70lbs worth of gear, so I think I've been made to endure this kinda crap. But that doesn't mean it's fun...
3) Nope - you won't look out of place. While some ultralight folks will point and laugh, you will
always find some freak with more gear than you walking to the hostel or the train station. And all of a sudden you don't feel so bad.
Despite all that, I'm going lighter from now on. After all, I go on trips to enjoy myself, not to drown myself in the luxury of my material goods, and I'm not as afraid of laundromats as I used to be...
That's the key - if you can bring just the stuff you can keep on reusing, you can cut your load down massively. My problem has traditionally been that my trips are big blowouts - I bring stuff that's good for the outdoors, good for sports, and good for nightlife. Not too efficient.. but I think I've learned to trim down my inventory while keeping a diverse itinerary. The easiest time I ever had, luggagewise, was when I took a medium-sized ballistic duffel bag. Rugged, easy to handle, and just enough room for the gear to put up with snow, hiking, and nightlife.
Taking all those experiences in account, I really think this Burton bag is going to be perfect.