PVE server of course.
Regarding faction - I think horde feels more unique and immersive. Going to wailing caverns, sfk, later rfk and etc just feels more cozy and the atmosphere is nice all around. Deadmines is very sterile and the whole human side dungeons and zones are just very dull.
I love some alliance races, maybe more than horde races, but horde has better feel in wow world.
I feel exactly the same way. Horde is Home for me!
Alli PvE ofc 
Iâll be going Alliance, since Blood Elves arenât available. Iâll likely play a warlock.
After reading through this thread, I think I will make a swap since no blood elf and go Dwarf priest or human nElf priest on a PVE server and engage in PVP when suits me.
A Pve character will be matched with other players of Pve/pvp? realms in the battlegrounds (WG/AV) but, in the new pvp zones⌠there would be only players of that realm?? (As a lot of people are saying, Pve is much more convenient, but it tends to have less populationâŚ)
Alliance RP-PVP all the way, but race and class still undecided.
Correct, battlegrounds are cross realm (PVE and PVP servers are mixed). PVP events are server only, but PVE servers are usually pretty well populated. I wouldnât be too worried about populations in general because due to the phases and level caps all players are packed together into just a few zones.
So you can safely join PVE horde, see you in Ashenvale. 
RP-PVP, for a horde guild who will push for Ashenvale battle ground in between raids
PVE Horde. I just hate pvp in general and want to play tank shaman.