Wow, where to begin!?
Back when I started out DMing, I would tell my players "Alright, this setting is loosely based on the Forgotten Realms!".
Of my Realms~
1a) Nowadays: All Gestalt, all the time. All the bigname NPCs are Gestalt, as are of course the PCs themselves. I've never truely understood people's hatred for Drizzt, Elminster, et al. They're just tools for the DM, to be used or ignored as you will. They complement my own homebrewed BBE(or G)Gs, not replace them. I've never seen Greyhawk fans dis the Eight, Vecna or Iuz! Which brings me to #2...
1b) None of the major Realms NPCs are statted out exactly like the books would have them (this is obvious if you look above, but would be true even if I wasn't using Gestalt so heavily). My Drizzt (for example) features a splash of Barbarian with Whirling Frenzy, just enough ranger to satisfy events from the books, a healthy degree of Rogue to justify his drow-stealth and ten levels in the Dervish PrC (it seemed a shame not to use it... it appears afterall to have been written just for him!)
2) Open Book. Toril is a ridiculously big world. I allow just about every WotC or Paizo released classes and a huge number of races in my Realms. There are alot of races I disallow for the simple fact that they don't bring anything new to the table. On a related note, major NPCs from other settings are paralelled in my Realms. Vecna is an immensely powerful Lich on par with Szass Tam or Larloch. Iuz is widely believed to have been a Chosen of Ixactu (spelling?) Xvim. Etc, etc.
3) The Isles of the Council of Worms lies somewhere off the SW Coast of Faerun. While this was much more relevant in 2e, I'd say about 80% of my Campaigns 1/2 Dragons (or otherwise Dragon-related PC/NPCs) have thier origins here.
4) The heart of the Anauroch Desert is actually a demi-plane version of Athas.... known as the the Deep Anauroch.
5) Several ancient Ruins (especially those related to the days of Myth Drannor and Netheril), Waterdeep (read: Skull Port), Evening Star, Menzoberranzan, Ched Nasad, etc, etc have portals tying them to Sigil. I've always had room for Planescape in my D&D!
6a) Gods. FR has a staggering number of gods. With my 2e notes, I have a list of which gods are actual, and which are just regional/racial aspects. Amauntor (spelling), the Sun God from Netheril is Lathander is Horus-re is... Pelor!

Azuth is an Aspect of Mystra (my Mystra is T/N), as is Velsharoon.
6b) Priests. Not every priest of every god is a fricking Cleric, Druid, Paladin or Favored Soul! In fact the majority of "lay priests" are expects with the appropriate skills.
7a) I don't use either the Great Wheel or FR's Cosmology, but my own. There is no Bloodwar between Demons and Devils (though reading Shemeska's SH, I might revise my choice here)... rather the "Blood War" relates to a struggle between two distinct types of "vampire"... Vampires and Nosferatu.
7b) Vampires are pretty much unchanged from the MM. Nosferatu are in some ways less powerful (no Energy-drain w/slam), but in others more powerful (they have a supernatural ability called Celerity that works as a constant Haste). Vampires are considered an abomination by Nosferatu, and are alot more common as they then to be more feral and alot less selective of who they make undead. One among the Nosferatu, a Drow Psion / Priest by the name of Krov'Xalyn Odran (originally a PC, later a BBEG) put events into motion to bring Kanchelsis (old deity of Vampirism from the 2e grey-cover Monsterous gods book... can't remember the exact name!) into the Realms Pantheon, using the Time of Troubles as a catalyst. This ended with a rather large showdown between Krov's forces (a considerable number of necromancers, vampire wannabes, vampires and nosferatu) and the PCs (all 13th-16th level... the lower leveled ones being triple-multiclassed)... at the time Krov was attempting to slay a very weakened Avatar of Kanchelsis in hopes of replacing him as the new God of Vampires in the Realms. (ahh... memories) All of this played out in a lengthy campaign in 2e... with a few "Nosferatu" PCs created using the Requiem Boxset. The vast majority of my Vampiric Villains have one way or another of entering the Underdark close at hand, in fact most of them fulfill thier entire existance down there... away from the hated Sun.
Damn, I had more to mention... but my memories of the Bloodwar campaign have pretty much chased everything else out of my thoughts. :/ Anyways, this should suffice as a taste of "My Realms".