Lala brit.... looks like this has just turned into debate about RYR...so hopefully this will help...
Strengths: simple (low cost) business model, yield management, single type fleet (B737), direct distribution (no travel agents), cash generating (cash in the bank is vital for an airline), well known brand, popular website generates advertising revenue
Weakness: Not too many to be honest (ignore the complaints about how they treat their staff, they are in business to make money), MOL can be a bit too voiciferous.... he needs to avoid doing a Gerald Ratner, business model can be weakened by high oil prices, secondary airports may restrict their market eventually.
Opportunities: Open Skies policy in Europe and opening up of eastern europe means there are plenty more airports welcoming them with open arms, franchising outside of Europe to generate additional revenue.
Threats: legacy airlines going low cost, new entrants to the market, other transport services eg rail..
Lots of other factors which affect all airlines: interest rates- most aircraft are financed, oil price, world stability eg war, terrorist actions affect tourism market, green campaigners (bit of a red herring in reality... we would all like to be green but who wants to travel less...), Government policy of airports (capacity etc).
The low cost model has proved extremely successful (RYR and EZY) being the biggest beneficiaries. Others have failed but the market is growing.
Try reading the Simon Calder book about the no frill sector.