I agree with most of what Yahtzee says,and he is right about some critical bullet points like the re usage of old characters in sequels,one of the main interesting stuff about good games is the interrogations that it leaves you with,for example take the first 3 halos,halo 1 was by far the most creative game of the 7(I think they are 7),while halo 2 and halo 3 spoil some of the mystery about the Halos and the covenant,the same can be said about good sequels like for example Hitman:blood money,which focus itself more on delivering new missions instead of keep running the same basis that contracts left(the whole Romanian cloning facility stuff.
However as a fan I must point out a game that often didn't screw up on its sequels,and guess what it is,yep ace combat which is by far the best game series ever conceived by human beings,the reason of why this game is so awesome is simple(take a note that I'm a fan and therefore whatever I say next gets into the layer of fanboysm).
Its all down to how the game is told,and how it encourage you during gameplay,I been playing ace series and the game hasn't move an inch from its basis stablish 15 years ago,but anyone who has played ace combat will have a different favourite game of the series, because they always change the way that the game is told,in AC2 you have your standard history of aces mercenaries taking down a coup,this happens in a strangereal place called "strangereal",but the evolution of both the concept and the game made the series make huge jumps,for example they forgot everything about what they did about top gun in ace combat 2 and decided to make an anime based story with AC3(only in Japan)but this aspect made the series evolve in many many areas like storytelling and concepts,and didn't recycle any elements from previous ace combat games,just gameplay.
After that come the best game in the series which are AC04,AC5 and ACZero(coincidentally all in PS2),the thing about these games is they way of their storytelling structure respectably,and how they decided to provide the story of each game with relation of the previous game but without using direct elements implied,just the place(stragereal) and some events like the asteroids from AC4 and the belkan war in both AC5 and zero,for example in AC04 you were an ace pilot in a normal war between the same enemies not represented directly but equal and with their own original background,in AC4 they told the story in gameplay with the radio charter and the mission briefing,while at the other side of the conflict an child tells the story from one of the affected countries and its relation with its enemy,and is really well put together and structured,is like portal 1 in this regard.
Then you have the Ac5 game with a really good story which suits more a movie,almost all elements from the past game are taken away,then using the existing assets in creating yet another good story using both gameplay,radio charter,briefings and cinematics,a bit over dramatic but way better that what you find in shooter or war related games these days,there is no patriotism and all that bullshit that plague all shooters these days(I should have address this before ,not involving the US faction in war games is really good for storytelling),and the story is well told and well produce,in fact one of the best productions I have seen.
And then there is my favourite game of all,ace combat zero,the 6th game manage to be the best game, although some people consider AC5 the peak of the series I sustain that Zero manages the top of all games,the reason for this is the perfect integration between all elements,everything fit together,the player decisions did matter in gameplay and everything is beautify put together to create and amazing ending that have direct correlation with gameplay and story,a perfect ending for a videogame,nothing is overdone and the story is simple,and it uses the gameplay to max the experience like any other of the previous games(similar to Half life 2 in this respect)and its uses also the perfect storytelling production without involving any of the previous elements,just the place and the gameplay.
So,I disagree with Ben Yahtzee in the aspect of "no games with good continuity",there are games with terrific sequels(HL2 for example)the only issue with such games is to find them or to have a good original concept unexploited for its income,something that have become an standard in the industry,apparently,and designers hasn't manage to find out best creative ways to create stories based on the existing gamplay instead of story(like project aces did the last 15 years until they sort of cock it up with 6 but never mind,maybe AC assault horizon manages to be good but one huge barrier is the fact that US in it).
And yep,I like portal 1 more,it was more "cheesy" somehow.