Correct me if I'm wrong, but I think that the first big crossover event was Marvel's
Secret Wars? That's the first one I remember anyway. At the time it seemed like a bigger, more complex
Contest of Champions.
I liked it for several reasons.
There were major changes. Ben Grimm/Thing stayed behind on Battleworld and She-Hulk joined the FF in his stead. Spider-Man got his black costume which incidentally turned into one of his greatest foes, Venom. (Not to mention Carnage, Toxin, et al.) New, lasting characters were introduced: Spider Woman, Titania, Beyonder. A villain, Magneto, became more sympathetic. The heroes who knew that Tony Stark was Iron Man learned that someone else (Jim Rhodes) was taking his place. More stuff that I can't remember.
Most importantly, the impact on the regular titles was minimal. You didn't have to read 8 issues of the X-Men (or whatever) if you weren't currently getting it. For instance, Spidey jumped on board the Beyonder's construct at the end of Amazing Spider-Man #251 - after a crucial battle with the Hobgoblin. Next time we see him (ASM #252) he's wearing a black costume that reacts to his thoughts, but we don't know why. And that was it basically, there were no other crossovers and the series (ASM) didn't reveal what the costume was until Secret Wars #8 came out. All of the other titles that were affected showed the same aspects.
The difference between Secret Wars 1 and
SW2 is drastic. There were what, 30 crossovers between ongoing series? A lot of those seemed shoehorned in or completely deus ex machina. Think the Avengers where the Beyonder just pops in and teleports Nebula to parts unknown, or the Hulk/Alpha Flight switchover where Marvel just needed an excuse to swap creative teams, or the totally obvious ploy to boost sales of a flagging title by crossing over into Micronauts. And more.
Furthermore, you didn't have to read all of the titles participating in the crossover, but you did have to read some of them to know what was going on. DC's
Legends event was similar to that. You need to read Cosmic Boy to know what's going on but you can totally skip Warlord if you don't normally get it. But how do you know what to get?
My favorite crossover events were the ones that they used to do with Annuals. I think Marvel was the first out of the gate with this as well.
The Evolutionary War went through every Annual that summer. You didn't have to buy the regular series plus they were jam packed with a ton of extra material - back-up stories, pin-ups, bio pages and even an ongoing background storyline.
Atlantis Attacks was the same way. DC eventually did the same thing with
Eclipso: The Darkness Within and
JLApe.
Another good idea I thought was the "Theme" Annuals. DC did that with
Year One,
Pulp Heroes,
Elseworlds and Marvel had an event where every Annual came poly-bagged with a trading card featuring a new hero debuting in that issue. Marvel also did a number of mini-series type stuff in Annuals like
Assault on Armor City,
The Korvac Quest,
Days of Future Present,
Spidey's Totally Tiny Adventure,
Terminus Factor and a ton more.
The reason I liked these more was that you really didn't have to get them all, but you might want to. I hated buying a comic that tied into a big storyline only to find out that it didn't have anything to do with the big picture and furthermore the writing and art was bad.
The original
Crisis on Infinite Earths was the reason why I started buying DC comics. Well, super hero comics anyway. I had already been into Sgt. Rock, Weird War Tales, House of Secrets, and a few others, but the super hero stuff eluded me. It was a little confusing, Earth 1, Earth 2, Earth Z - I could never figure out who belonged where. I liked Marvel because it seemed like a more cohesive universe so I liked what Crisis did even though I didn't really understand it at the time.
So, I guess what I'm saying is that every crossover event doesn't have to be a "Huge Giant Shift Nothing Will Ever Be The Same Again" type thing. Those are nice but just do them every couple years or so and intersperse some other fun stuff along the way.
Hardships often prepare ordinary people for an extraordinary destiny - C.S. Lewis