A very interesting topic, and I've been interested in cartoons since my youth.
The 80's were an oddball period for cartoons. On the one hand, that was the decade when many of us were young and so many of the properties are nostalgic for us. The positives were that the notion of using anime on a network was very limited, and American companies were still coming up with lots of original ideas and franchises. The fact that TMNT, MASTERS OF THE UNIVERSE, TRANSFORMERS (which was an anime I believe), and INSPECTOR GADGET keep coming back in relaunches, movies, or both is testiment to that. However, the big limitation was content. Censors were incredibly strict in terms of violence and maturity for cartoons, especially once SMURFS took off and every network believed that cutsey animal characters were the "new age" of animation. And cartoons were usually made for very young audiences; watch a typical episode of NINJA TURTLES from the late 80's and you can easily tell that it's made for a very young audience, and is very kiddy and jokey; even moreso than TMNT: FAST FOWARD is. It's also far less violent. You could argue the same with one of the holy grails of 80's cartoons, SPIDER-MAN & HIS AMAZING FRIENDS, or even Spidey's short lived 1981 show. Or Hulk's 80's show and so on. The 80's were a time of lots of creativity and originality, but they also had severe limits on what they could accomplish, and shows that pushed the envelope too much had short lifespans. The 80's ended with a lot of enjoyable franchises and characters, but with the solid foundation that cartoons are for kids, and should teach them lessons. And have kiddie, animal characters.
In the 90's, something happened. Anime was becoming more well known after 1989's AKIRA was seen in some theatres (VOLTRON, BATTLE OF THE PLANETS and STAR BLAZERS also had become cult hits), but there was also the notion that cartoons could strive for more universal appeal, like motion pictures, to tag many demographics; kids and teens, some adults, etc. Naturally, BATMAN: THE ANIMATED SERIES is the show that many point to as the one that started this trend. It was an immediate hit and so the 90's were full of showing willing to ape it. The censors did ease up a bit on violence since the 80's, but it depended on the network; Foxkids was unduly harsh on some Marvel shows, especially SPIDER-MAN. You had GARGOYLES, SUPERMAN, BATMAN BEYOND, X-MEN, THE TICK, and decent 2nd seasons of IRON MAN and FANTASTIC FOUR, and a good first one of INCREDIBLE HULK. However, by the end of the 90's, many franchises were already out of steam and so attempts at new ones; SPIDER-MAN UNLIMITED, SILVER SURFER, AVENGERS: UNITED THEY STAND flunked. Plus, networks had become addicted to anime; POKEMON had become a breakout hit and suddenly anime was a household term. It became very easy to dub anime than to produce original shows, especially since many times the network themselves didn't pay to translate or get rights to anime, but a 3rd party (or a smaller company connected to it; i.e. Funimation is connected to the WB). Plus, these shows got ratings (especially since many imports imitated each other). And amidst this, CARTOON NETWORK appeared as an area where shows could escape strictor networks in terms of violence and maturity; DBZ, heavilly editted on network TV for the Frieza saga, became better on cable. And JUSTICE LEAGUE fled there. The beginning of the 90's were willing to push the envelope but by the end, ratings and merchandise retook the priorities again, especially as franchises ran their course.
During the 90's, of course, THE SIMPSONS became a hit and set the stage for cartoons to include heavy adult, mature, and sexual themes & violence...but only for network situation comedies. So you had THE CRITIC, FAMILY GUY, et al. But that stuff never happened for non-comedy cartoons, except on rare HBO or MTV stuff like SPAWN or THE MAXX.
So, into the 21st century and cartoons are in a mire. Most networks, even Cartoon Network, rely on imported anime for 95% of their schedules. If any network has more than 1 show that's Western based, it's a miracle. The advantage is that over the years, animation technology has improved by leaps and bounds, as well as outsourcing it. The fewer cartoons a Western company produces, the higher the quality of animation (HANNA-BARBERA would produce a good dozen or more shows in their heyday so they all looked cheap, for instance). Some standouts continued to push the envelope from the last decade, especially JLU and the newest stab at TMNT. But after a few seasons those shows ran their course. Now the pendulum has swung back in favor of lighter shows like THE BATMAN, TMNT: FF, LOSH and TEEN TITANS. There's more violence and maturity than in some 80's fare but not as much as the innovative early-mid 90's. And CARTOON NETWORK is a devil in sheep's clothing, putting any Western show on haitus for months at awkward times to pimp out anime more. Any show that goes there for less restrictions has to deal with a show that is constantly given the short end of a stick. Modern shows now want to imitate anime's stylings but with few exceptions, want to imitate the character depth or strict storyarc focus that anime has; TMNT and at times TT or JLU are the only exceptions. In 2006-2007, the older 21st century standouts are gone and so we have more kiddier shows again, albeit more violent and mature, and with better animation, than a lot of 80's fare. At this point I feel that the audience and the creators actually are more than ready for more improved mature shows like from the 90's, but it's the suits and networks that got scared (probably from "mothers angry about everything" type groups) and aren't ready to advance beyond archetype.
That's just my take. Feel free to disagree.