I want to get from Buenos Aires, Argentina to Santiago de Chile by land. What is the recommended route and what are the alternatives? Aprox. how much it costs and how long does it takes? Where can I book tickets and find schedules? What are the main problems that can be encountered?
There are some direct buses from Buenos Aires to Santiago de Chile. For example, CATA Internacional runs a daily bus, on weekdays only (Mon-Fri), which leaves at 17:00 from BA (Retiro) and arrives next day at approximately 12:30 (= duration ~19.5 hours). The price is $AR 900 / USD ~105 (semicama without service) or $AR 1050 / USD ~125 (cama ejecutivo) as of Nov 2014.
Besides the Argentine CATA, two Chilean companies run direct Santiago – BA buses on some days of the week: Pullman del Sur (details) and Buses Ahumada. Prices on Pullman del Sur from CH$ 55,000 (USD ~100).
Alternatively, take the bus to Mendoza, Argentina (12+ buses daily; 13-16 hours, USD 60–110 depending on company and comfort level), located quite close to the Chilean border, and then continue from there to Santiago (about 8 buses daily, 6-7 hours, USD 40-65). The mountain pass and border crossing is called Los Libertadores.
I haven't taken this trip to Santiago myself (only to Mendoza), but I'm fairly sure that the bus is your best (and maybe only) bet, and that you won't encounter any major problems. The coaches in Argentina and Chile are comfortable and reliable. (For a long trip, I recommend investing in cama suite or whatever the best option is called, to be able to sleep better.) You can see timetables and buy tickets (with credit card) online at Plataforma 10, or you can visit the Retiro main bus terminal in Buenos Aires [map] for the same.
By the way, Mendoza is a nice place known especially for its wines, and some of the highest peaks of the Andes nearby, and stopping there for a few days wouldn't be a bad idea.
Edit: updated for November 2014. The prices in local currency keep rising a lot (for example, Mendoza–Santiago trip went from 120 to 230 pesos during just five months in 2013). The Argentine peso is weakening at the same time, however, so visitors do not feel it as badly.
There are several bus companies that will take you. The easiest solution, once in Buenos Aires, is to head to Retiro bus station. Go up to the second floor, and to the section for 'west bound' buses.
There will be several companies. El Rapido, Andesmar, Cata, Tur Bus, and many more - it's an extremely popular route.
As Jonik mentioned, the Plataforma 10 website is easy to use, but note that it doesn't cover all the bus companies.
If you have the time, I highly recommend stopping in Mendoza, for two reasons. One - it's my favourite town in Argentina, with the best wine around. Can recommend the All in Monkey Mendoza Hostel, and they run wine tours, or you can use Mr Hugo's bike tours. The steak is also the best I've ever tasted - El Fuego's restaurant is cheap, and better than any high class restaurant steak I've ever had.
The second reason is that it allows you to get a morning bus out of Mendoza. You want to be travelling to Santiago during the day - one, because the border crossing at 3am in the snow is VERY COLD (Depending on time of year), and two, the scenery is spectacular. Seriously, spectacular. It takes 8 hours (I've done the crossing three times), and is amazing - a great day trip.
Sorry about being brief on my last post - a tad more info here now. After going Retiro teminal in B.A. on Friday I was told that there would be no coaches until Monday due to disruption in the high passes around Mendoza.
The good thing is I get a couple more days in the wonderful city of Buenos Aires however I have found this page:
It gives a current status of the main pass in English and Spanish.... There is a Tunnel Open-Closed icon just above the twitter icon on the right and current info in the main content.
Hope this is of use....