Can a eponheart pact go in to daggerfall covenant just like on wow a alliance can go into horde starting zones? or is the only pvp in cyrodill?

Should go here http://tamrielfoundry.com/development-faq/ Great place to start and will save you having to start new threads with questions that have already been asked tons of times.

Cyradiil only.

PvP is Cyrodiil only, but is that the only places differing factions will cross paths? For example, can a high elf Aldmeri Dominion character explore Covenant or Ebonheart regions? Or will you have to have three different characters to explore the whole world?

once you reach 50, yes

At level 50 you can go to the other two faction zones, but you will NOT see other factions in there. The only time you will ever see other faction players is when you are in Cyrodiil and that's it.

Nope, megaserver technology has a lot of awesome benefits, but it also means catering to the lowest common denominator - players who would choose to play on PVE servers if given the choice.

It does at least mean that Cyrodiil will be a permanent blood bath

what do you mean by (but it also means catering to the lowest common denominator)?

no, indeed its Cyrodiil only

I get the impression that Cyrodiil plays a far greater part in the game than many people realize. It is divided into three distinct regions, each being themed for an alliance. And not only does each have multiple NPC towns but multiple public dungeons.

The idea of going to other starting areas with lvl 50's is simply to allow people to do some quest content without rolling an appropriate character. Its was an afterthought to appease pve only players more interested in the experience from single player TES games. But the rest of us would be funneld into the larger portion of the world known as Cyrodiil. It is bigger than any of the Alliance areas and contains much of the endgame content.

The only legitimate concern I see with this games design is not allowing pure PVE'rs the chance to opt out of pvp when they try and access Cyrodiil. Personally I think the design is a good one and if people are not willing to fight for the right to be there they simply don't have the right to be there. But that's just my view and probably not shared with the majority of gamers who only play Single player games and stuff like Farmville or SimCity or whatever.

Meaning you can't force more hardcoe playstyles into the game when the server is shared by all player types. Other MMOs would break these playing preferences up (PVE, PVP, RP, etc) - ESO is lumping everyone into the same big group, so rather than upsetting PVE players it's best to force everyone to play by their preferred rules. Far better to have PVPers asking on the forums for open world PVP, than PVE'ers declaring they're cancelling their subs.