While I highly enjoyed Fallout 3 in many ways (primarily as a survival/shooter)..... I remember:
Reilly's Rangers
Outcasts
Talon Company
Slavers of Paradise Falls
Tenpenny Tower
Etc.
And how there were almost no quests that made the different factions interact ....... there was so many cool places with so little reason to go to them or return to them. (my opinion)
I hope Fallout 4 has fleshed out factions that have quests against one another (can't do them all) or at least lots of quests to perform to help/aid the factions you want.
One of the things I like most about Fallout:NV was the amount of quests you could do for every faction - House, Legion, NCR, Powder Gangers, even Khans and Fiends and minor factions often had one or two..... it made the world feel interactive and alive to me.
I realize the game is almost done - but I had to post since I'm playing FO3 again in anticipation and felt the old familiar ache of wishing the many cool faction HQs and members were more fleshed out with dialogue and especially quests.