The thing is that each fighting game has it's own ways of being played. Each game has it's fair amount of memorization, knowledge, recation speeds, etc. etc. needed.
SSBM is nice in the sense that it is indeed easy to pick up, and if you want to get good, then it takes a lot more time to get better. But it's the same excate way for every other competive fighter out there. It is all based on *HOW* the game is played.
Let's take Tekken for example. It's button masher friendly, so moves are easy to pull off. But, you think a button masher is going to succeed in playing agianst someone who actually takes the game seriously and competes in tournament level? Of course not, the button masher will be doing random moves he does'nt know, and therefore not properly adjusting to the situation, while the actual player will know what to block, how to block it, if he get's a successful launcher, know if he can take them to a wall, or whatever combo.
Not to mention juggles in Tekken are character specfic. Just like in Smash, just like in Marvel vs Capcom, and especially just like Guilty Gear XX.
Other games have some really tough situations where fast reactions are *indefinetly* needed. A good example would be in Marvel vs Capcom 2. Trying to consistanly block Magneto's triangle jumps is an extremely tough feat. If you are on the ground, Magneto has 4 different directions he can hit you, on your upper half, on your lower half, on your upper half on your back side, and on your lower half on your back side. And each of Magneto's attempt takes about 10 frames. 10 frames in a game that runs about 55 frames or so per second equals to about having 1/5th of a second to react to each of his attempt. That's less than a quarter of a second. Now think about blocking correctly for about 3 or so seconds if he is using an assist which can force you to continue blocking. And one simple light attack from him can easily either turn a match around, or get a win in a match.
Another strategy would be called footsies. Footsies play a huge part in say Street Fighter III: 3rd Strike. A footsie game would involve trying to keep an opponent within his/her maximum range that their attack so that if they throw out you can act on that mistake, throw out your attack, and land big damage. Ken vs Chun-li is a footsie oriented match up. Chun-li has superior range, and also can turn a match around if she lands one simple low medium kick. A strategy Ken needs to do is by baiting Chun-li into throwing out that low medium kick so that he can throw out his medium kick and land his super, therefor giving him the edge in the match. The thing however is that Chun-li's low medium kick is about 5-6 frames (not sure the actual data) and the game runs at 60 frames per second. So agian, that's about 1/7 of a second you have to react to that medium kick, and punish it accordingly.
Granted these techniques are'nt the only ones in each game, they play an important part for each said game.
Agian, each game has it's share of what is needed to play each game. Smash I feel is more based on execution and mindgames. When edgeguarding, if you are on the one that's off the platform, you have some options that will allow you to make it back, the more options, the more ways you have to try to trick your opponent so that you can safely make it back. Another thing is exeution, missing one simple L-Cancel can hurt you big time for the simple fact that you can be punished a lot easier. But that does'nt mean these two things are the only skills required to play Smash. But little things like memorization is needed for Smash. Otherwise, you are just a combo dummy since you won't DI correctly.
But agian, each game has it's own ways of being played. Obviously, if you are going to ask SWF if Smash is much harder to play and get good at, you will probably get a lot more people saying that Smash is indeed tougher. But out of all of the Smashers that I personally know, they "play" other games like Tekken and what-not, but their main game is indeed Smash. And while the "play" those other games, they are not good at them, so why should someone asking if Smash is harder to learn take their word for it? They don't know how to play anything else, and don't have any remote knowledge of what competive Tekken/Guilty Gear XX/Street Fighter/etc. etc looks like. But the same thing can be said for someone that plays SF/GGXX/etc. etc. and does'nt know the first thing about Smash other than all you have to do is just push forward B for special moves and call it a noobie game. Even Technomancer's post was leaving out information. He gave a good idea on how the spacing works, but left out footsies, reversals, meaties. If he just recently picked up ST and/or CVS2, he still has things to learn. You don't get at these games over night. Especially since ST requires a vast knowledge of character match-ups. As for CvS2, it all depends on what groove, character, and what your fighting agianst. There are way too many examples to give right now so I'm not, I can though if someone wants one though.
All I can really say is that agian, each game has it's own way of being played. Some games and some games are easier than others. If you want to get good at Tekken 5 (the original version, not the new one Dark Ressurection) you just need to pick Steve, learn what few things are needed, and bam, you will be winning a lot of matches in no time. But not all games are like that, Smash is'nt, Street Fighter is'nt, GGXX is'nt.
Although I will say that Virtua Fighter is the hardest game to learn currently. Nothing else comes close. Every big thing, every little thing, and everything else in between matters to the finest point in that game.