World Traveller is ordinary coach; and the seat pitch is rather tight, the seats are pretty narrow as well. World Traveller + is much more comfortable for a long haul, but the price difference is pretty steep. Essentially, the only difference in those two classes is the seat pitch and the width by an inch or two. Food on BA is always rather good, and I believe alcohol is still free, but drinks in general don't come 'round often; it's hard to get a cup of water when they FA's are not making normal rounds. (BTW, asking for ice will get you one cube, and that only if you ask.) If WT+ has not sold out when you check in for the flight, it is often possible to buy an on-the-spot upgrade for a lot less than the WT+ would have originally cost.
The most important thing to be aware of on non-US carriers is that the weight and size of cabin luggage (carryons) is much more tightly enforced than it is by US-based carriers. I believe BA's carryon limit is 12 lbs.