There is a theory out there in the blogosphere that JLA/Avengers is not all it could be, because it's "nothing but fanservice."
To which my response is, "Yeah, well, that's kind of the point of intercompany crossovers are for. What else did you expect?"
I suppose part of it may be because it came out at a rather unfashionable time for that sort of thing--late 2003 was the end of the big Marvel creative renaissance, and the genuine revitalisation was giving way to its own type of excess and there was a general sense that this kind of old-school superhero stuff wasn't very hip anymore and, as I remember reading reviews at the time that were sort of "Oh well, let's look at this, then . . .it's OK for what it is. *sigh* . . .I guess."
In a sense, it's kind of the end of an era for both JLA and Avengers, as Busiek's more backward-looking run was soon to be supplanted by Bendis' vision for the Avengers franchise and Morrison's big epic JLA paradigm would soon be undermined by . . .well, Joe Kelly's inexplicable love of The Elite, really. This is kind of the last hurrah for that mid-90's moment, I'd say.
That aside, and with seven years of hindsight, we can say that while JLA/Avengers is fanservice, it's mostly very good fanservice and manages to weave the insanely tangled and convulted continuity headaches that characterise the publishing histories of these two teams, provides the requisite level of spectacle, and also gives you more than a few Easter Eggs to enjoy in the bargain (seriously, there's probably a web page out there with exhaustive annotations on all this stuff). While on one level that is that woeful bugbear of "comics about other comics" as it occurs in a controlled self-contained environment story-line wise, it just about gets around it.
Not that it's perfect--the forcible inclusion of Hal Jordan and Barry Allen in the third chapter and their amount of screentime turned out to be a bit of unintentional foreshadowing, both in their reintroduction to the DCU and the clumsy way in which they're dropped back in and forced importance is . . .well, that also turned out to be a premonition too, dinnit?
Anyways, enough pre-amble, let's get to the story. After a Crisis on Infinite Earths homaging beginning we get to to cases pretty quickly. Krona and the Grandmaster agree to play a game, wherein the JLA and Avengers compete to collect a number of famous McGuffins. The first issue wisely concerns itself with setting things up--we see the JLA fight Terminus (who once again goes out like a bitch--Terminus must be the most oversold and least successful Big Threat Marvel ever devised--seriously, there has never been a Terminus story where he wasn't played for a giant-size schmuck) Meanwhile, the Avengers fight Starro, there's some travelling to each other's respective worlds and we get our first good bit--Heroes in the DCU are idlolised and super-famous, Marvel heroes are outsiders and viewed with suspicion by the general public. This also helps to build in some animosity between the two teams and better justify the obligatory "heroes fight until they figure out it's all been a misunderstanding, thing." The Avengers think that the JLA must have set themselves up as figures of worship as an oppressive force, and the JLA think the Avengers are dangerous loose cannons.
The two teams don't acually meet until the last page, and then the fight's on. It's a good use of pacing to use the first issue to build the stakes up to that point--these are the two biggest superhero teams out there and there'd been decades of buildup to seeing them meet, so the moment when they finally face off should, ideally, be a Big Thing, and the first shot in their battle is a hell of a cliffhanger to leave things on (especially since there was a bit of a delay between the first two issues and that made the anticipation even more unbearable) but you actually feel the catharsis when the fight begins and escalates in issue #2
The second issue concerns itself with the battle between the Avengers and the JLA and the subsequent quest for all the McGuffiny Basically half the issue is taken up with the main casts having a rumble, and during the treasure hunt for the second half, we get various reserve JLAers and Avengers getting some face time whilst Batman and Captain America try to figure out what's really going on (we also get a cool image of Darkseid with the Infinity Gauntlet, which was a great "oh shit" moment). Finally, we get one last throwdown in the Savage Land before the cliffhanger of this issue--Krona goes nuts and attacks Galactus, and the Grandmaster reveals the purpose for the various McGuffins and seals everything up in a nice little pocket universe . . .
. . .the results of which we see in issue #3 (Did anyone ever name everyone who was on the cover? Are they still sane?) which starts as a succession of riffs on the old JLA/JSA team-ups that were a staple of Justice League of America for . . .ever, really and this is pretty much the "all fanservice" portion of our program as damn near everything that is possible to fold into an invented shared history for both teams, including (insanely) the aborted JLA/Avengers team-up from the 1980s. The "shared history" bit soon unravels as it becomes plain that Krona is planning to shred the universes apart and only by going back to the way things should be can things be set right.
This is set up to be a hard choice for both teams to make, but I was more amused that it involved more or less the characters accepting that they'll have to endure more than a few shitty retcons and/or dying. This is where Hal Jordan and Barry Allen get shoehorned in, to the slight detriment of the story--considering they soon get folded back in the main plot, their inclusion here at best is only to add more weight to the "hard choice" that must be made, but when you consider that their replacements show up again in the last issue, well, it makes the whole thing seem like a digression and makes their successors look like second-stringers. Which, for a certain segment of the comics-reading population, I guess they always will be.
Anyways, it's all to set up the big cathartic moment where the Justice League and the Avengers put aside their animosities and team up to fight Krona. It's a great punch-the-air moment too, not least of which because it's been earned over the course of the story and there's been the weight of expectation that they would, ultimately, team up.
And issue 4 delivers on that as damn near every Justice Leaguer and Avenger that ever was teams up to fight every Justice League and Avengers villain George Perez felt like drawing (SPOILER: A lot) and it pretty much delivers on the promise of grand-scale action, ties up neatly and just . . .really works. I'm glossing over the specifics of the plot here basically because 1) it's better if you read it and 2) really, the plot's just a vehicle for the spectacle of the crossover, and really, no surprise there.
Ultimately, JLA/Avengers exists to create a broad canvas for two of the linchpin superteams in the Big Two, tell a reasonably coherent story, provide enough big moments to justify itself, and then get off the stage. It doesn't blaze any new trails, but then if you're doing an intercompany crossover, it's not the greatest time to try and blaze trails--that way leads Deathmate or Spawn/Batman--and it succeeds in what it was designed for with aplomb and efficiency.