This game is different. Playing previous games may help your execution (your ability to successfully do the moves you want to do, for example, turn around up-tilt) or character familiarity, but the way the game is played is very different. Offhand, here are a few examples:
Shield: Shield stun is very low. What does this mean? Well in say Melee, some moves can only be punished by a shield grab or maybe an extremely fast aerial out-of-shield. Now, you have other options. Simply letting go of the shield after blocking an attack is so much faster that you may now do something like jab combo or forward smash if the opposing move has slow enough recovery. Speaking of those jab combo, there are many situations where a full jab combo is an optimal option, something that wasn't used much in the previous titles.
Combo's / Followups: Hitstun is less, I'm sure you've noticed, this means that other than the fact that there are less combos, there are more options for the defending player. One option off the top of my head, is simply jumping out of a followup. Why is this different? Well in Melee if you have the opportunity to jump out of a combo, you may not want to because if you jump AND get hit, you have lost your air jump and that is fatal. This brings another point, many characters can recover from far under the ledge even without their air jump so now you don't have to be afraid to lose it.
Approach: Many moves are unsafe on whiff and on block. IMO, due to the removal of L-canceling, even more moves are unsafe on whiff and on block. You cannot blindly charge in with aggression without proper knowledge of your landing lag or spacing. Doing so will get you punished by my first point: the Shield.
So there's way more stuff right? Those are just a few things. To the everyday non-gamer, all Smash games probably look the same, but as you can see, they are not. Try to find out what works and doesn't work in this game (and for your character, since many characters can't do everything). It certainly helps to play this game with a fresh open mind. Luckily you can play online, read forums, talk to other smashers, and watch videos.