It's simple. The point of the WTO is a mechanism for "bargaining" down trade barriers--and enforcing the bargains, once struck. Without the WTO, the US might say that it will drop tariffs on wine with the EU, if the EU drops tariffs on beef from the US. That's a bilateral deal, and is good for both countries, but there are a couple of problems. Firstly, small countries can't bring much to the table to bargain with, and tend to get left out. Argentina, a major exporter of beef, would be hurt by being "shut out" of the beef market in the EU. Second, even large countries might not always find a particular offer worthwhile. In the above example, the EU might consider beef tariffs too "valuable" and refuse the deal, and the US might not have any other plums to offer.
The WTO solves both problems, by letting small countries band together, and by letting large "packages" of reductions be put together, so that everyone gets something good out of the deal. This is called a multilateral deal, and although quite complicated, is good for everyone involved. There is a much simpler alternative, however. Instead of complicated treaties and negotiations, a country could simply unilaterally drop all barriers to trade. This would actually be good for the country (the benefit to consumers will always far outweigh the cost to big business), but would be better if other countries did so as well, and so most countries prefer to "trade" tariff cuts for matching ones in other countries.
That's the main reason why countries want to be in the WTO - particularly developing countries, which are desperate for lower tariffs on agricultural products and textiles. The 3rd world countries know that the EU would never let their hugely pampered farmers suffer without good cause (although EU consumers would be quite happy, but tend to be less visible than the farmers). The WTO is therefore the 3rd worlds best bet: If they're lucky, they can trade something fairly unimportant to them (removal of restrictions on foreign ownership of telecoms, let's say) for something vastly beneficial, such as lowered tariffs on goods they export. This isn't always easy, even with the WTO. Consider the current breakdowns, which have little to do with protests, rather, the developing countries are sore that the 1st world hasn't done what was promised during the last round. Even so, the choice is stay out in the cold, with no chance of ever having enough clout to get any important barriers lowered...or enter, and have a much better chance.
China has been working very hard to get into the WTO, despite the fact that it entails a massive shakeup of their entire economy, and a real chance of political instability. Why are they so keen? Because it's easily the best, and possibly the only way they can manage to remove the massive barriers that have been set in front of them, and China needs them removed very badly. China has a massively growing population, and either the economy at least matches it, or a nuclear power with the worlds largest standing army, several territorial disputes with other nuclear powers, and several rebellious provinces (one of which is ALSO probably nuclear armed)...goes BOOM! No, I think we need to keep those peasants in poverty myself--fatter subsidies for the steel workers! Note that the protesters weren't Chinese.
As it happens, the current head of the WTO is from New Zealand, and a 1st world country, the population is less than 4 million, and it is heavily dependent on agricultural exports, mostly wool, cheese, butter, and so forth. Not a particularly important country--which is why New Zealand is such a strong proponent of free trade. We don't ask for an advantage, we just want a fair go...which is why all my friends are as puzzled as I am about the protesters in Seattle. Fair trade? That's what the WTO is DOING.
Finally, the WTO is there to enforce agreements once struck, but please recall it was still your politicians that first had to agree. Once the US has agreed not to ban tuna imports, it can't then turn around and ban them, however popular or worthy the cause is now seen to be. The fault is that of shortsighted politicians, not the WTO.