I don't think it's so much that "They changed it", but "How they changed it" with an emphasis on the word how. "Oblivion with guns" isn't just a tired old insult, it's pretty accurate description - and whether or not one thinks Oblivion is a good game, has no greater relevance here, Fallout should've remained Fallout and not turn into a TES clone.
Oh, how so untrue that is. In every respect...
...apart from you saying ....
"Oblivion is a good game, has no greater relevance here," .......
As for "Oblivion with guns" as some have said, that has always been a ludicrous thing to say, as has been shown.
Re the topic.
I played the early Fallouts and they were great, I yearned for a next version of Fallout, but wondered if it would be as good as I remembered the old Fallouts to be, at that time.
Bethesda excelled themselves in that respect, improving about every aspect of the early versions, which made me wonder why I thought that the early games were so good. Must have been because, way back at the time, they were a ground breaking type of play, something new.
Anyway, Fallout has now evolved into a far superior game as in Fallout3, yet is still in keeping with the kind of mature play that the old Fallouts were.
There are those who seem to be nit-pickers of really minor details, minor detail changes that are usually more about giving the game a very good balance of play, if you thing about it, as Fallout3 is.
Those who favour board game combat play, as the early Fallouts had, are unlikely to favour Fallout3 for it's real-time play. Board-game combat play can be fun for a short while, but is so flawed for games with the size and amount of content of games of today.
But each to their own choice.