The most controversial issue about natural wines - and the reason why they're of interest to so many people - is how much sulphur is used in the winemaking process. Purists say none should be used at all - although there is naturally occurring sulphur which will be detectable even in unsulphured wines. Many others reserve the right to use a small amount at bottling to stabilise the wine.
The problem is we have no idea how much sulphur winemakers are using - and won't unless the regulations change or someone decides to break ranks and put the amount they've used on the label (which I wish they would). The phrase 'contains sulphites' covers everything from miniscule to massive amounts. No help to those who react badly to sulphur and have maybe given up the pleasure of drinking wine as a result.
If you want to know more let me recommend this excellent post on morethanorganic.com which explains the issues with admirable clarity though they obviously have an agenda.
For a more technical approach check out my colleague Jamie Goode's site Wine Anorak here and here.
It's a subject I'm sure I'll be returning to but what's your view about sulphur in wine? Do you think a wine needs to be unsulphured to be classified as 'natural'? Should winemakers put the sulphur content on the label and can this be done in any meaningful way? Do you yourself suffer from allergic reactions to sulphur?