I probably should've just made a new post instead of editing my previous one on this page, but hopefully you see it anyways Dahbomb. One thing I forgot to mention is that as your percentage goes up, so does the amount of hitstun you suffer as you get hit.
You're gonna see a lot of Fox because: his Smash attacks are quick and strong, particularly his up-smash which can kill people at curiously low percentages. He also excels at killing people early because he can knock you off the side of the stage and use his Reflector (down+b) to knock you away from the stage. Fox/Falco both have them and they can reflect projectiles, but they can also hurt people if you're close enough to them when you first activate them. Fox's is notorious because it sends the victim flying sideways even at a low percentage. His blaster doesn't cause hitstun, but it's extremely fast and can rack up damage quickly.
Falco is the Ken to Fox's Ryu. Same moves, different properties. Falco has his "pillar" strategy where he racks up damage by using his Reflector (which knocks the opponent vertically into the air instead), then jumps up and uses his down-air to send you flying back down to the ground. If the opponent is bad at teching when they hit the ground, he can keep doing this for a good long while until he feels like killing you. His down-air is nasty without this strategy too, since it shoots the opponent downwards pretty quickly. Factor in hitstun and you can kiss a stock goodbye.
I don't know a whole lot about Marth and Shiek other than Marth is your standard "i'm a sword character, therefore I'm way too good" character, so hopefully someone more knowledgeable than me chimes in. Also on Fox/Falco, too.
You're going to be seeing a lot of stage counter-picking. Obviously some stages have less hazards than others, but there's other things to consider. Some stages are actually larger than they appear since you also have to take into account how high the ceilings are for each stage.
Jungle Japes, for instance, has a deceptively high ceiling.
There's a lot more, but as I mentioned already this is just basic stuff. There's other, smarter people in the thread who can also answer questions in here if you've got something else you wanna know.
Also, fuck martha.