You know, it's not the 2D isometric vs. 3D FPS that's the issue. That's a more an evolution in market tastes than a gameplay choice, and was bound to change. (Though, yes, there are still several 3D isometric tactical games out there --- Jagged Alliance 3 is coming along nicely).
The issue is that the story doesn't come close to the original.
FO1 was a darkly ironic take on a retro-future-that-never-was. Dialogue trees branched like a mutated rainforest, and NPC's actually had something to say I was interested in hearing. So far (and I'm pretty far along), I haven't seen that in FO3. Writing is ploddingly adequate, at best --- and the bland delivery of the voice-acting isn't helping --- and for the most part, painfully boring.
FO1 had player choices that actually led to consequences you couldn't take back. Some of the NPC's only let you speak to them once, and then never again --- so you better make the right choice. FO3 has never let me screw-up a dialogue interaction with an NPC --- even when I tried to: Just talk to them again, and everything's fine, mate! Oh, you were a total wanker? No worries! Hell, in FO1 one of the funniest things you could do was create a character with an low Intelligence, and then watch how NPC's interacted with you! Better Intellegence / Speech led to more dialogue options. In FO3, it's all the same --- the dialogue tree is all right there. Wouldn't want to make anybody think that choice = consequence, would we? Nah. They might not like our game, and go back to playing GTA...
But what is really glaring, is how the Perk / Skill system has been gutted. FO1 pretty much made you specialize. There was no way to be a jack-of-all-trades and get away with it. You couldn't max out all yr stats, and Perks were only available every 3 levels. Not to mention, certain Perks (OK, mostly Traits) came with negatives associated with them. (Fer instance, Night Person: In FO1, the bonus you got at night, was actually a negative modifier during the day). FO3 just gives you a Perk every level because everybody likes free stuff right? So the more the better --- and since nobody likes being told There Ain't No Such Thing As A Free Lunch, all Perks are now consequence free, positive aspected only. Yee-hah! Every level-up is like X-mas! Again, don't want to disappoint anybody so they keep on playing our game.
Fallout 1 made you work for it --- and if you wanted to try different play-styles, you had to create different characters. From Gun-Monkey to Diplomat, they were viable, but you had to be either/or. Fallout 3 just feels like it's all layed out on a big silver platter, no effort involved. (But then, that's kinda what they did for Oblivion, where you could join every Guild, and complete every quest, without having a single ability related to what you were accomplishing). Nothing like writing RPG's for the ADHD generation, I suppose...
But hey, I actually like Fallout 3! I'm actually enjoying it for what it is: Another FPS/RPG hybrid, just like Oblivion. I just wish Bethesda had called it something else...
Oh, and spent some of the cash blown on graffix on just one decent writer.
