Well, I'm aware that I don't want to derail OP's thread, so I will try and be brief. Basically, I was convinced when I started reading early church documents, that the first believers were quite convinced that they were eating Christ in the Eucharist, which was central to their worship. The churches that have had, as a feature of their worship, a very big focus on keeping very close to the earliest traditions, like some of the really persecuted Orthodox in the middle east and so on, all still held very firmly to this belief.
Now, the Orthodox don't call it transubstatiation. That's a term that came about through the more forensic, western approach of the Roman Catholics. But the concept of actually physically consuming Christ is very much there.
It's a hard doctrine to get to grips with, no doubt. But, for me, it's one of the most indefinably powerful parts of my faith - the Almighty God loves me so much, that not only did He come to earth as a human being and live a difficult life and die a difficult death, He comes every day to continue to be physically united to me. He is not a distant being, He is absolutely immanent.
Couple that with a very basic idea of 'you are what you eat' and if I consume the all-loving God, there's a better chance of me becoming more loving etc. it's pretty central to me.
Wrt closed communion - personally I don't have an issue with it - if you don't believe the stuff I've just outlined, why would you be keen to receive communion in my church? I don't receive in other churches because I dont believe the same thing as they do about the ceremony, and part of the idea of communion is that you agree to what it us you are consuming.
Hsat, that's just communion. I'm more than happy to engage in many ecumenical endeavours - with other Christians and other faiths. It's just there are always going to be parts of our beliefs that we cannot share with others, and that's just reality. As long as we don't rip into others on that basis, it's ok.
(Hope I haven't gone on!!)