Monkey Island 2 felt very different from the first game imo, and not in a good way.

For all of that, Warrior Within is a much better game than Sands of Time. Sand of Time's combat is almost painfully simplistic going back from Warrior Within, the sound quality (especially of dialogue) is poor, and it's very buggy, especially when it comes to Sand powers like winding back time. Too many times in that game I had the time meter cut out for no reason while rewinding, wasting my sand. And then there's times where it would only rewind to a shortened point where I would inevitably die.

It took me several attempts to want to complete Sands of Time.

War never changes.

Excellent responses.

Special relativity was a great remaster of Galilean relativity, and its sequel general relativity really fleshed things out nicely. The second law of thermodynamics is also a lot more interesting than the first, which just boils down to a statement of the conservation of energy - pretty derivative stuff.

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The streamlined, sexier version.

Almost none. There are non-essential story elements that you can pick up on if you play them in order, but it's not remotely important to the main story.

Day 6 when god realised he had made a very bad mistake...

When franchises change hands, for example:

-Far Cry 2
-Mechwarrior 4 (It even upsets me now remembering such bad game, after the jewel that was Mechwarrior 3)

Yeah, Jak II reminded me very much of Warrior Within in this regard. It was also this kinda forced attempt to make the game "more mature", giving up the previous title's very personality.

I guess I got lucky by playing Warrior Within first. I ended up getting positively surprised by playing Sands of Time afterwards rather than getting disappointed by Warrior Within.
Well, I didn't quite feel this way about Half-Life 2 in this regard. I mean, Half-Life 1 also already had tons of environmental puzzles, many of which involved pushing boxes around and stuff. I didn't feel like adding advanced physics really changed the spirit of the game - although they may have focused a bit too much on them.

The change of the setting and the change in tone was certainly very weird to me in Half-Life 2, though, as was the way they decided to continue the story. They went from Stargate / Aliens / X-Files, with some black humour, to some weird post-apocalyptic adventure for the whole family (hey look, a cute robotic dog buddy!)... set in some Eastern European city because why not.

Although to be fair, in Japan Dynasty Warriors 2 started a new series. For instance Dynasty Warriors 8 is titled Shin Sangokumusō 7 there. It's only in the West that the title suggested that DW2 is a straight sequel to DW1.

Edit:
Oh yeah, and I just remembered that I had the same problem with the very game I started playing just after Jak II: Darksiders II.

I was actually very disappointed by the game when I first played it briefly after it was released. They turned this really tightly designed and expertly balanced action adventure / slasher game into an almost ARPG with lots of loot and stats and a quest structure - due to this the difficulty curve is all over the place and the game is very easy most of the time. And instead of epic cutscenes you usually just get a bit of dialogue with NPCs. And there's a lot of fetch quests. But most jarringly they really changed the setting and tone a lot. One of my favourite aspects of Darksiders 1 was the really original and ridiculously epic post-apocalyptic setting where every location feels entirely different. In Darksiders 2 you end up in some cold fantasy realm where everything is kinda generic and samey, at least as far as I've played it. I have yet to find a location here that makes my jaw drop due to how beautiful or original it is.

I'm really surprised nobody has mentioned the abomination that is Sacred 3 yet. They had a successful open world Action-RPG series and turned it into a linear mediocre co-op brawler without loot and without quests and only minimal character development to appeal to a wider audience.

Didn't quite work out as expected since the fans of the first two games weren't happy about the changes and brawler fans had a lot of better games to choose from.

Everyone tried their hardest to forget. ;)

Personally I didn't even mind the genre change or mediocre gameplay that much, but the writing and "humour" was so cringeworthy that it would have made any game unbearable to play.

What about Dungeon Siege 3? I hear it doesn't have much in common with the other games anymore either? And Dragon Age 2 might also qualify to a certain extent.

Has anyone yet mentioned Dune 1 and 2?
Between Dune 1(Adventure game + slightly strategy element) and Dune 2 (The grand father of RTS genre). completely everything had been changed.

Except Dune: The Building of a Dynasty (also known as Dune 2) was never a sequel of Dune - those two games were developed simultaniously by two different companies with their personal take on the subject.