Accommodation - be prepared for lots of nights of staying in dorms. Most are packed with bunks, without much space for your gear; you'll be in close quarters with your fellow travellers, often with just two or three showers for the Exodus group and whoever else is staying! The craic was great in the dorms on my trip, so be ready to take the rough with the smooth and go with the flow. It's only for one week, and you've a lovely hotel in Chamonix to look forward to at the end!
Walking - although there are no summits on this trip, be aware that you're carrying all your gear, which is why it has the C/D grade. I personally found it closer to a C than a D. I'm a 36-year-old day/weekend hillwalker and I found it fine. I definitely found it helpful to train up for self carrying, by loading up my bigger rucksack and doing some training walks in advance. Obviously the fitter you are the easier you'll find it, and therefore the more you'll enjoy it.
Weather - although the typical photos of the Alps in summer are all blue skies and flowery meadows, I did this trip in mid July 2008 and spent half the week in low cloud with no views. Just be aware that you may get a day or two of limited views due to low cloud. Those Alpine pastures aren't green and lush for nothing... it does rain! Just prepare for rain and sun.
Alcohol - be prepared for tiny measures of wine in Switzerland. You'll literally be served a small tasting glass a third full and it will cost you about 2 to 3 euros.
Smoking - we had two smokers in my group and they couldn't find anywhere to buy cigarettes for most of the trip. Strangely none of the little supermarkets or bars sold them until we arrived in Chamonix near the end of the trip. Take your week's supply with you!
Several days of the trip are in Switzerland and although the shops, cafes and hotels you'll encounter almost all take euros, the rate of exchange between British pounds and Swiss francs is much better than the rate between pounds and euros.
We didn't encounter any ATMs until the penultimate day of the trip, when we arrived in Chamonix. Bring enough to last you until then.
Also, Chamonix is very expensive.