I live in Sydney, and often go home to the UK, and when i do i order new clothes online, have them delivered to my parent’s house so all i have to bring is a carry on bag. (Although today a friend who works for the aussie customs and immigration told me they are more likely to stop and question people with just carry on for security concerns).

This bag: Eagle creek Rincon 65L
http://www.eaglecreek.com/bags_luggage/adventure_travel_packs/Rincon-65L-10081/
65L of well constructed awesomeness. More importantly, you can carry it on an airplane. The ‘daypack’ goes under the seats and the main bag goes in the overhead. It’s slightly larger than the boxes they have to check your stuff, but almost no one does anyway. If they stop you just tell them…it fits the correct way (umm…parallel to the normal vector of the overhead, sorry) in every plane I’ve ever been on except those really big international ones that have four seats in the middle row. In those you just have to put it in the other way.

Bring a couple of carribeaners to hand stuff off of your bag

Those recyclable grocery bags are nice if you want to bring food around with you.

The flex lock like you recommended is great, as are those tiny TSA travel locks. The flex lock is important for locking the bag to yourself on sketchy buses so you can sleep a little better.

Exofficio underwear IS awesome. Buy it.

I picked up an HP DMZ1 laptop and it was a great purchase. $450, great specs, 95% size keyboard, etc. It’s really well designed and I am very pleased with it. Plus it’s tiny 11.6″

A petzl headlight is absolutely crucial. I have one which also had a red LED on it which is really good for sleeping in dorm rooms at hostels because it doesn’t wake people up as much.

The microfiber towel you recommend is great, though I hate the feel of it. Big fluffy towels are one of the things I miss most.

REI makes great travel shirts and pants.
Handsome, breathable shirts with UPF and secret pockets: http://www.rei.com/product/794446/rei-sahara-tech-long-sleeve-shirt-mens
And they make some nylon pants (Not convertible!) with additional zip up pockets near the sides.

The Panasonic Lumix DMC-ZS1 is a great travel sized point and shoot that takes VERY impressive pictures (pretty much as good as a point and shoot gets) and is a great price. I bought mine years ago, they’re probably on a different model number by now. They’re popular.

A pair of headphones is important if you’ve got a laptop.

Bring a journal.

Travel socks are nice, but buy a few extra pairs of regular ones there.

You can carry on nail clippers and cartridge razor blades, but not a pocket knife. BUT you can buy pocket knives pretty damn cheaply when you land. I’ve even seen combo knife, spoon, fork, and bottle openers for about $5 in several countries.

Bring good earplugs. Those Israeli silent ear were too pricey for me (I lose them) but the highest rated box of cheap earplugs on amazon are excellent and cheap. I brought a whole bunch.

A kindle is a sweet way to fit 300+ books in your bag. Even better you can download tons of free books that are out of copywrite from sites like the gutenberg project.

Platypus water bottles are awesome. The take up no space when empty.

I brought a cheap quad-band GSM phone and just popped in local pay as you go sim cards if I felt I needed cell phone service (though usually I didn’t)

A compass is really useful in cities when you get out of public transportation and are disoriented

Carry photocopies of your important documents and scan some to evernote/dropbox

Bring a sleeping mask

Athletic tape and ziplock bags are awesome

Carry purel

You probably don’t need a guidebook

And never, ever be without toilet paper.