A lot of people are having difficulty differentiating what a shared universe is, which is totally understandable.
The amount of franchises that are currently afflicted with sequelitis has led to an unprecedented amount of reboots/sequels/prequels since the 21st century, its no wonder something as specific as a Shared Universe is successful, let alone recognized.
If the Marvel Cinematic Universe is presented as the lead for this question, it is because it is the prime example of such a strategy; in no part, as you've identified, because of its unprecedented success.
The MCU is a narrative continuity that was conceived as being constructed by overlapping texts, at the very point of its inception. By operating in a shared environment, Marvel are able to replicate the success of their Comic Publishing legacy by presenting an audience with several movie franchises, that are able to make narrative sense individually and in relation to each other.
They are developed simultaneously, commissioned concurrently, and released in a chronology that (for the most part) converges into a larger, ongoing narrative.
In contrast to this, a sequel (or prequel, or re-quel...whatever) is usually commissioned and created one text at a time in part of a linear narrative: the success and response to one directly dictating the direction of the next.
The MCU is by some way the most successful example of a shared, overlapping cinematic Universe; but it is not strictly speaking the only example.
There is, indeed, a president for tactical overlap, but these texts exist only in gradation to the example presented by Marvel.
There are other attempts, that construct shared universes, but with varying degrees of intention and success. As you've already identified, some are throwaway jokes, parody and cameo references; but others share the intention of the MCU (and in some cases predate it by some time!), but have a narrower scope.
Star Wars and E.T the Extra Terrestrial
This is the weakest and least ambitious 'shared universe' connection, but (given its impossible to disprove) still an example. Depending on who is telling the story, George Lucas and Steven Spielberg either wrote the E.T character as being a Star Wars alien, or (much more probably) simply traded referential in-jokes to each other.
Sites like Cracked.com seem to keep pushing the issue, typically because it's often written as 'Mindblowing' to attract a certain type of green-dreamer. I don't want to 'harsh anyone's mellow', so I'm acknowledging it despite it's unlikelyhood.
Withnail & I and Wayne's World 2
This one slightly disrupts the 'shared conception at inception' part of the definition given above, but is still worth a mention due to its novelty.
Withnail & I introduces a transient character named simply 'Danny', who behaves, dresses and speaks in 'a very specivic vay, man.'
This character is played by Veteran Brtish TV actor Ralph Brown, who some people may immediately recognize as Queen Amidala's royal pilot from ...yup, you guessed it! Star Wars:
Ralph Brown has a lot of versatility, and has been cast in incredibly diverse roles. There was, however, a character who popped up again somewhere else; albeit under a different name: Del Preston.
Anyone who has seen the two characters can vouch that its pretty undeniably supposed to be the same character. The change in name is (given his 'vock 'n voll baby' personality) about as significant as Ziggy Stardust suddenly showing up as David Bowie.
Wayne's World 2, whilst a comedy, seeks to make this connection explicitly; meaning to make narrative sense, it must be set in the same universe.
Personally, I wouldn't submit to the film ever making total narrative sense; as that would mean Wayne's World is also set in the Terminator Universe.
The Matrix Universe
The Matrix actually attempted to execute a shared universe, but it's success was largely overlooked due to the fact it pursued a transmedia strategy; creating the Universe across multiple mediums, but only pursuing the 'Main' narrative in Cinema.
Whilst The Matrix itself was commissioned in isolation, the remaining elements sought to simultaneously develop and overlapping narrative structure across Animation, Comic Books and Video Games (VG).
There were developed concurrently, interlocking with each other and directly informing the content of their other constituents.
Theorist Derek Johnson uses this franchise as a case study in 'Searching for the Origami Unicorn', in which he interviews the producers of these separate elements and outlines the strategy (conceived personally by the Wachowski's) of creating a literal 'multimedia' franchise.
The fact that it attempted to straddle a VG franchise in some ways makes it more succesful than Earth-199999, which is so far yet to make a VG that competently contributes to its wider universe.
The fact that the film explores each narrative in a separate medium excludes it from being a 'Cinematic Shared Universe', but it's strategy still represented a significant shift in the way media is produced; for an audience of fanboys, it represented a truly rewarding experience for anyone to complete the narrative through it's continuation: to see how deep the rabbit-hole goes.
The Three Colors Trilogy
Krzysztof Kieślowski is not a name you'll see on M&TV often, so its no surprise he hasn't been listed yet.
Regardless, his contribution to cinema is unique in that it probably represents the earliest incarnation of a shared universe: in respect to the definition presented here.
The Three Colors Trilogy is a set of films that, just like the MCU, was conceived to be an overlapping and 'cross pollinating' set of narratives. All three are separate, standalone narratives that can be consumed in isolation and still be entirely coherent.
They also, like the MCU, converge into a single isolated moment, with each separate narrative contributing directly to our understanding of the final events.
These films are so successful at this level of convergence that they (in their overlapping entirety) are often referred to as 'masterpieces'; and with good reason.
They may lack the scale and production value of the MCU, but they beat it 'Shared Universe'
release strategy by some 30 years, which surely counts for something?
Honorable Mention: Hot Shots: Part Deux and Apocalypse Now
