Common New Player Mistakes

I saw a good post on this topic on SRK recently, and thought that the topic would be a good candidate for a note here, that can be easily updated and supplemented. If you're a new player to SF4/SSF4 or to Capcom fighting games in general, there are a lot of things that you might find yourself doing (either consciously or unconsciously) that are bad habits. The intent of this post is to help you avoid doing these things yourself!

Wakeup Specials/Supers/Ultras: So you've been knocked down by your opponent, and you start to panic. He's bearing down on you, preparing to do who-knows-what as you stand up, and you know that your [Shoryuken/Flash Kick/Headbutt/Cannon Spike/whatever] is good at getting your opponent off of you, and may even be invincible on startup to boot. Resist the urge to use it. Too many new players go right to their Dragon Punch (or similar move) as they get up, and all your opponent has to do is block your DP, then follow it up with a huge combo for free. If you do this repeatedly, you're handing the match to your opponent on a platter. If you do use such moves on wakeup, use them sparingly, only often enough to make your opponent nervous about attacking you on wakeup. Often, the best thing to do on wakeup is to simply block or backdash, but be wary of throw attempts.
Overuse of Specials/Underuse of Normals: If you're playing Ken, you might want to use that fierce Shoryuken a lot, because it's invincible, does a lot of damage, and lights the other guy on fire. The problem is, like many special moves, you're left wide-open to a big counterattack if your move is blocked. Normal moves (that is, moves performed by pressing a single button, or holding a direction while pushing a button) are quite a bit safer, are usually pretty fast, and are generally the preferred option to use as "pokes." Pokes are moves that you use just out of your opponent's range, to bait a response that you can punish or to land a hit that can lead into a combo. Dancing in and out of attack range while doing this is called "footsies" and is a core aspect of Street Fighter play. Through footsies, you can counter-hit/punish your opponent's moves and land your own and follow them up with combos. Special moves (generally) are too slow and too unsafe to allow you to use them for this purpose. Special moves are best used when you're fairly certain they will hit (e.g. an armor breaking move against a focus attack), at the end of a combo (e.g. ending a Ken combo with a Shoryuken), or for other specific purposes (e.g. zoning with fireballs to keep your opponent away from you, or to anti-air a jump-in). Avoid throwing them out without a specific purpose in mind. If you are throwing out random Shoryukens from half screen because you don't know what else to do, you are making this mistake.
Random Ultras: Ultra combos look pretty cool, but they're generally very easy to avoid if done while your opponent is just standing there, and they usually leave you incredibly vulnerable afterwards: you'll usually eat your opponent's Ultra if they have it available. Research your character's Ultras and learn how they're supposed to be used: in combos, as anti-air, as a close-range mixup option, etc. Try not to use your Ultra in ways other than the recommended way. The main exception would be in situations where you use your Ultra for chip damage (forcing your opponent to block the Ultra which causes minor damage), but relatively few Ultras are suitable for that purpose.
Predictability: A lot of new players learn one combo, and then spend the entire round trying to land that combo, often by jumping in over and over. Such predictable patterns are easy to counter, and you should mix up your actions to keep your opponent guessing. Occasional jump-ins are just fine, but footsies (as described above) are also a good way to start a combo on your opponent. Mix up your attack patterns with throws from time to time. Which leads to...
Underuse of Throws: Throws are an essential part of Street Fighter play. They are a good way to mix up your attack patterns, by breaking off a combo at varying times and moving in for a quick throw when your opponent is not expecting it, and they grant an untechable knockdown which lets you follow up with a variety of attack options as your opponent gets up. Consequently, your opponent will probably be trying to throw you often as well, so be aware and be ready to tech at any time.
Bad Punishes: One of the most important things to learn when picking up a new character is to learn what their best punish combos are. Punish combos are combos to use when your opponent is wide-open for a counterattack, such as after a blocked Shoryuken or whiffed Ultra. Every single subpar punish (such as a sweep or throw) is free damage that you're just throwing away. Punish combos can usually be found on any combo list, or on any community web site.
Abusing Lag-Only Tactics: Using tactics that only work online due to latency, such as mashing DPs during your opponent's combos in case they miss a link. If you plan on seriously bettering yourself as a Street Fighter player, it's best to get out of this habit early, because such online-only tactics are easily punishable in an offline setting via frame traps and simple baiting.

Anyone who is able to edit this post is free to do so, and I will do so over time as well if I think of anything else that belongs here. My aim is to make this a good all-around primer for new players who are looking to improve their game and rise above "scrubby" tactics.