Most of the time, I think the point of expansions is to improve replayability. Most games are fine without any expansions, but might get boring after you play them enough time. How many 'enough' is varies greatly on the game, but the approach I'll take toward the question is what games have relatively little re-playability without expansions.
Citadels is one that plays fine without the dark city expansion, but Dark City adds SO much new variety its hard to imagine playing without it. With just the base there's one possible set of 8 characters; with the expansion there's 256 possibilities with 8, and an extra 256 if you play with 9. There won't always be a gigantic change when you switch some characters, but there's definitely at least 20 different combination that all make the game completely different and require a total recalibration of strategy. The later editions of the game mostly come with the Dark City expansion in the box.
Settlers of Catan is another where the expansions add a huge amount a re-playability. The base has a significant number of different maps, but as with Citadels the expansions increase the possibilities of the island exponentially, not to mention giving some completely different strategies. Ending up with smaller surrounding islands with Seafarers will definitely change your building plans, and you'll probably think differently about your military if you have to worry about being attacked in Cities and Knights.
Countless more games have similar situations, like Dominion, Three Dragon Ante, Carcassonne, Pandemic, and probably at least 200 others. IMHO any good expansion should add a lot of re-playability.
Are they satisfactory without the expansions? Definitely. Are they really complete? That's a matter of opinion. Is the Dominion base set complete because it has 3,268,760 possible kingdom sets of 10? Or is it not complete until you have at least one big expansion and 10,272,278,170 possibilities? Or 2 big expansions and 828,931,106,355 possibilities? Or all the first 4 expansions and the 3 promo cards and 6.20885664 × 10^13?
I totally agree that there are some games that aren't very good standalone. My point is that the definition of completeness if very subjective. :D