For me, a "true" Fallout is all about what Kaotiker said. "stimulating in the totally uncensored, completely hopeless, bitter and cynical world they portrayed, full of people gone insane from the events that occurred, of course coupled with the sense that you were a real actor in that world". He said it brilliantly so there's no point restating the same thing. So I guess Fallout fans CAN agree. To extend a bit, it's a lot about exploring this world, interacting with it in different ways (remember the dialogue for a low intelligence character?), and getting to know (and love) its lore.
Because, I must say, Fallout has one of the greatest pieces of lore I've ever experienced. That is why F3 by Bethesda didn't work. Not to say they can't come up with touches of genius every now and then and that the engine is bad. I just hated the way they dismissed the assistance of ex-Fallout dev team members, and it really showed. Fallout 3 was a 3D Fallout Museum of sorts, with mostly bland story, characters and dialogue, with little sparks of genius left in the side quests and locations. What many newcomers enjoyed in the main quest was way too bombastic and out of place to fit the Fallout lore. Fallout isn't about the "big deal", it isn't an "epic saga", but a huge, brilliantly painted veil that encourages you to stare at the tiny details that happen to fit so well in the whole.
At least Obsidian got it right with New Vegas. So right, I wish they had developed the initial Fallout 3 as well, with Bethesda just working to improve the technical side, engine and such.
Therefore, not being an RPG, Fallout Tactics cannot be a "true" Fallout, because as a TBS it lacks the possibility to develop the core elements that made F1&2 the legends they indeed are. It's a gorgeous, tight and challenging Fallout-themed TBS and worthwhile for both TBS and Fallout fans, since F1&2 had turn based combat and it's unlikely that any of their fans didn't enjoy it.