The 10 tips:
1. The Torres del Paine Circuit trail itself is only moderately difficult, the most challenging section being the John Gardner pass. There are no issues with route finding and even moderately experienced hikers will have no issues hiking the full circuit without a guide.
2. Hiking times are estimated very conservatively in most online/printed sources. For even moderately experienced/fit hikers the Circuit may easily be hiked in 6-7 days and the W in 3 days.
3. Pack for four seasons as snow may be encountered at all times even in summer, and use lightweight gear. A full backpack including four-season gear, food and camping equipment, need not weigh more than 10 kg. Unfortunately high-quality lightweight gear is not cheap.
4. No need to carry food for the full eight days as food (both meals as well as supplies) may be bought at all the refuges. It can be difficult to get food into Argentina from Chile, thus the safer option could be to stock up in Puerto Natales. I had no problem with my freeze-dried meals at the land border, however.
5. Use sunscreen even in cloudy weather. The sun is very strong.
6. No need to carry more than 1 liter of water as water sources are abundant.
7. Lonely Planet Hiking in Patagonia (from 2009) is very outdated as well as out of print. The Cicerone Torres del Paine guidebook may be preferred. The National Park provides up to date maps.
8. Tighten your backpack. Even backpack cover may blow away in the Patagonian wind.
9. The main challenge of the Circuit is crossing the John Gardner Pass. On the other hand it is the greatest day out of the entire circuit: Technically it is straight-forward, however snow and strong winds may be encountered at all times. In brief: 1. During the ascent you can see the pass high up. If you loose the markers, just walk in this direction. If the pass is obscured by snow or fog: Wait it out 2. The winds on top of the pass may be ferocious. 3. The descent may be slippery, however ropes are provided.
10. Two sets of vertical ladders have to be climbed crossing two ravines a couple of hours before Refugio Grey. The most difficult moment on the hike, in my opinion, and the only moment with exposure on the entire trek.
What I did:
Solo-hiked the Torres del Paine circuit including the W hike with full camping gear (10 kg) over Christmas 2014, with the following itinerary:
El Calafate (private bus)-Laguna Amarga-Refugio Las Torres-Puesto Serón-Refugio Lago Dickson-Campamento Los Perros-Refugio Grey-Campamento Italiano – Valle del Francés (return)-Refugio Los Cuernos-Campamento Torres-Torres del Paine Lookout (return)-Refugio Las Torres-Puerto Natales (bus)
General information on the Torres del Paine Circuit and W:
Adventure Alan provides comprehensive practical information for both the Torres del Paine circuit and W trek.