In spite of its simplicity, opponents of Sola Scriptura so often misunderstand it. Is it the total exclusion of any and all traditions? Does it mean if something doesn't come from scripture, it's not true? Are there any gaps/ambiguities in scripture, and if so, does that in any way harm Sola Scriptura? To illustrate this, let's examine one simple phrase: "Please get me a glass of water".
If your house guest asks you this, it's unlikely you'll think about it too hard. In your head, however, you will answer a number of questions you don't even realise you've asked, starting with where will you get this water? Realistically, you're not going to head all the way to the corner shop and buy bottles of fresh water if your kitchen is just down the hallway.
The next question you might ask is "which glass?" If you have a whole cupboard filled with glasses, do you look for a specific one? Do you bring all 20 glasses to your guest and ask "which one do you want?" The specifics of the glass aren't apparent in the request, so you can pick any one, they're all equally valid. Nevertheless, it seems obvious that a watering can is not appropriate.
It can be reasonably assumed that the glass ought to contain a fair amount of clean water. Why would you pick up the nearest plant pot, let some water drip through the drainage holes, and bring your guest about 5ml of the result?
Most of this is immediately obvious from the statement alone. You don't need to phone a friend and ask "what does my guest mean by 'please bring me a glass of water'?" And if your friend does weigh in and says "oh, actually, you should bring them some lemonade with an actual lemon slice", you can shrug them right off.
Right there, you have four paragraphs detailing the depths of one simple sentence, all of which your mind will blow through in less than a second. Notice the variation of depth, however. Each consideration ranges from obvious to pointless.
The first thing we considered was where to get the water? Because the water is the important element, the where to get it is no consideration at all. In the same way, a lot of things in our faith, we put way too much thought into. How long should a Sunday service be, if we even have one? What, specifically should we wear, in and out of church? Should we celebrate this holiday, or eat this food, or give this amount to charity? The Bible tells us to do various things. It tells us to meet with other believers, it doesn't specify that this has to be every Sunday for X amount of time. It tells us we must dress modestly, neither too revealing nor too flashy, it doesn't say we should wear a particular kind of suit or dress. Food and holidays, there is an entire chapter (Romans 14) dedicated to saying it doesn't matter, because as Colossians 2:17 says they're a shadow of things to come. And whence commeth the mythical tithe? The Bible says to give as you purpose in your heart (2 Corinthians 9:7), it does not say 10%, or indeed any specific purpose. So, at the end of the day, the how we obey is not as important as that we obey.
The same consideration rings true for which glass do we choose? We put a lot of emphasis on God's plans, which we don't know, and not nearly enough emphasis on just plain obedience. Would God's plan be in any way hindered or delayed if we just did what He said? Of course not. God will fulfil His plans even if you disobey, and may even, if you are so inclined, use your disobedience. So why do we spend so much time asking God "should I marry this person?" or "should I take this job"? The answer is simple: If there is wisdom in doing so, and you may do so without breaking a direct command, you may take any opportunity. As Mike Winger once put it regarding the romance question, "it's not so much that there's 'the one' so much as 'a one'" (quoted from memory, actual quote may differ). As for your job, unless you have been given a direct command from God, why would He care if you're a car salesman or a police officer?
It was also fairly obvious that the watering can was not appropriate. It's clear from the spirit of the statement that if a glass was not available, something similar, like a cup or a goblet, should replace it. In the same way, the spirit of the law is fairly obvious in scripture. If it ever seems difficult to obey the letter of the law, and it often will, seek to obey the spirit. We see Christ illustrate this with David and the showbread (Matthew 12:1-8; Mark 2:23-28; Luke 6:1-5). We could look to something like baptism and ask whether we necessarily need to baptise in a river? It's impractical to expect everyone to be baptised in the same river as Christ, but it's clearly not appropriate to sprinkle babies, who cannot even believe, and call this baptism.
Clean water is a context-based assumption. If a guest asks you for water, it can be reasonably assumed that they intend to drink it. They aren't going to bathe in it, water the flowers, or splash it in their enemies' faces. This entire analogy is based on a single sentence and minimal context, but it's ridiculously rare for scripture itself to be so ambiguous. You may see the occasional throwaway verse, though no examples of such come to mind, but the overwhelming majority of the time, the Bible gives plenty of specifics. More than enough for the average reader to not even need to make reasonable assumptions about purpose, as purpose is explained.
Finally, I mentioned the friend coming in with babblings about lemonade. This, of course, corresponds to sects which reject Sola Scriptura. The Pharisees were an excellent example. As Christ said to them, "All too well you reject the commandment of God, that you may keep your tradition." (Mark 7:9). When God commands something, it makes sense that man has no authority over it. Traditions are nothing compared to God. We may well have them, but the instant they conflict, or even add to, God's word, we are actually guilty of sin.
Most people who believe in Sola Scriptura will agree with everything I have just said. Notice the complexity within the simplicity. Does Sola Scriptura mean scripture is completely clear of any and all ambiguity? No, scripture is often explicitly ambiguous. Does it preclude tradition? No, so long as tradition does not conflict with, nor add to, the word of God, it is permissible for individuals to hold to them. Not to bind them upon others, as that is adding to the word of God, but to keep them for one's self? No problem. This is a very simple concept: Study scripture, take in its context, stick to the letters where possible, stick to the spirit always, and let God supersede all men, individually, or as a whole.
Unfortunately, as scripture warns us, there will be false teachers. They will claim to be leaders, they will cast doubt upon the scriptures in some form. As Irenaeus wrote in Against Heresies, when heretics "...are confuted from the Scriptures, they turn round and accuse these same Scriptures, as if they were not correct, nor of authority, and [assert] that they are ambiguous, and that the truth cannot be extracted from them by those who are ignorant of tradition. For [they allege] that the truth was not delivered by means of written documents, but by word of mouth."
This is the very picture of Sola Scriptura. Or rather, the denial thereof. I was most amused recently, upon quoting this to a Catholic, when she replied, she said "...I mean, you just quoted it right there "truth cannot be extracted from them by those who are ignorant of tradition."" Irenaeus says heretics say truth can't be extracted from scripture by those who are ignorant of tradition, this Catholic basically says "amen, truth cannot be extracted from scripture by those who are ignorant of tradition", what does that make her by Irenaeus' reckoning?
Just as Irenaeus identifies heretics by their denial of Sola Scriptura, so also does he identify their motive: Confutation from the scriptures. Scripture is correct, authoritative, and not ambiguous, and thus truth can absolutely be extracted from them who are ignorant even of true traditions. False traditions? Of course truth can be known by those who reject a lie. So when a Catholic comes along and says Mary is our advocate, we can absolutely throw 1 Timothy 2:5-6 at them. When a Mormon comes along and says "as man is, God once was, as God is, man may become", we can definitely throw Isaiah 43:10 back at them. When JWs come along and say Christ rose in spirit only, Luke 24:39 is a wholly appropriate response.
Unless their claims to authority are true.
With no claims to authority, all people are free to learn from scripture. But if these groups truly had authority, would scripture not at least seem consistent with their beliefs? Would it be so easy for Christians to declare to these cults "this is what the Bible says", but so hard for these cults to likewise say "actually, it says this"? Denial of Sola Scriptura comes not from a flaw in the doctrine, but from its exposal of the flaws in false doctrines.