Afghanistan is one of the "poorest" and least "developed" countries in the world. Even though lots of effort have been done to change the country It is not the ideal place for nomad, especially western (or any other country involved in a war there like Russia) passport holders. Kidnapping, landmines and sicknesses connected with poor health system must be taken in serious consideration before going there.
Afghan roads are not always in good conditions (though depends where) and are often populated by Talibans, military or simple bandits check-points.
This can be a problem, especially if you don't speak Dari (or Farsi) and are not well dressed/mingled among locals (bandits will be a problem anyway since they rob anyone..)
Avoid hitchhiking at night and in the southern part of the country.
- If you are coming from Iran and going eastwards, biking or hitchin, the road till Herat should be safe (always check current info) to continue to Kabul best option are fliying (quite expensive, around 100$) or taking a bus (best from Abdalli company, they have deal with talibans who stop them less than others..) defnitely cheaper (around 25$) but also definitely more risky, passing through the whole south and Kandahar; don't get out of the bus (excpet for praying as a good muslim) for any reason until you have reached your destination and dress and behave as a local..
- the road between Kabul, Kunduz and Mazar should be hitchable of course with great precautions.
- Bamyan lake region should be safe though it is not at all the road leading there from Kabul, in 2012 controlled by Taliban, take a bus and plenty of precautions.
- Wagah corridor in the northeastern part of the country, between China, Tajikistan and Pakistan has become a quite popular tourist destination for westerners, it's quite safe, deserted and beautiful
Local hotels could be expensive, especially in Kabul. There is a fairly big couchsurfing community in Kabul made mainly by expats working there plus some wealthy local. In Herat and Mazar there are only few hosts which could be quite active though. Be always careful accepting spontaneous hospitality, keep in mind that, in spite of people amazing sense of welcoming and kindness you are still an ambulant wallet, for kidnappers as for anybody bringing you to them; and you are in a country that still haven't get completely out of war.
Be careful of the condition the food is prepared and stocked..
Wifi should be available only near western hotels or military installation (probably not accessible by many). Local internet cafes might be available in main cities like Herat, Mazar-e-Sharif and Kabul.
Citizens from all countries need a visa to get into Afghanistan.
A tourist visa is not too hard to obtain, you need again a letter of recommendation from your embassy (it might be a little harder get it since they may not really want you to go there, e.g. Italian embassy in Tehran doesn't give it) but without it, if you talk kindly to the Afghani functionaries they may accept your request also without it. It normally costs 40€ and is ready in 3 working days, they keep your passport at the embassy.