IIRC, there was a survey done towards the end of last year which found that a British passport gives you the most access to other countries, in terms of not needing visas/being able to acquire visas etc (From memory, South Sudan is the worst.) One of the things about having a British passport for me, though, is that most of the countries I need to go to are the ones with bloody expensive and convoluted visa processes for the British - and free/inexpensive ones for Canadians and Kiwis 
I believe both Greece and Turkey require dual nationals to do National Service: and iirc, the only other nationality you can hold alongside Russian is Israeli - though that may have changed now. Didn't the US only use to permit Canada and the UK at one stage too?
If any of you are - like me - bemoaning the fact you are only entitled to a single nationality, don't forget that as British passport holders, we can hold more than 1 UK ten year passport, as long as you can prove you need it for travel to incompatible countries (ie Israel and Syria/Lebanon/Libya/Sudan/Saudi
) or travel a lot for work and need to passports because of applications for visas etc I have held two for over 20 years- you need a letter from your employer to take them out initially, but after that, you can renew both simultaneously without: if you try to do them separately, though, you need to provide a new letter etc The UK is pretty unique in this respect -afaik, the US, for example, will issue you an additional passport, but it's not a full one and is time limited.