Just a couple of cautions. First, remember you cannot re-enter the country of origin and leave again. This becomes an issue with UK-issued tickets because of the importance of LHR within the Oneworld scheme of things in terms of routing. It may not matter to you depending on your itinerary, but intra-Europe travel can become a nuisance later if you can't transit the UK between, say, N. America and, say, northern/eastern Europe.

Second, you probably know that RTW tickets are priced differently from one country of origin to another. With today's exchange rates, a DONE5 purchased and originated in Poland is almost £1000 cheaper than one purchased in Britain; it's around £750 cheaper if purchased/originated in Sweden. Aside from the obvious price advantage, starting/ending in some other European country means you could transit in London later if you need to pass through. I'd also mention that ticketing through AA might save a lot on fuel charges, and both WAW and ARN are served by an excellent General Sales Agent (GSA) for American, Tal Aviation. The combination of ticket price savings plus fuel surcharge savings could well be very significant in total cost terms. Certainly it's greater than the price of a short flight to Warsaw or Stockholm.

The down side of starting elsewhere in Europe is that it automatically obligates two of your four available European segments - one to get from Poland/Sweden/wherever to an intercontinental gateway from which you can travel on to Oz/Asia/Americas/Africa etc. and the other to get back at the end. This may be a big enough drawback that it's worth the £1000++ in extra cost, but that's up to you.