Among "average Joe" musicians, the ability to solo is the most sought after part of rock and roll. (that, and post-performance hookups, well deserved hangovers, and money)
In all seriousness, if one were to test the soloing ability of all of the proficient guitar players of the world, the absence of skill in this area would be shocking. There would be the occasional diamond in the ruff, but outside that very little would be impressive to the trained eye and ear. Take a sniff around YouTube with the word "solo." More than half are young, naive, often arrogant pseudo-musicians who attempt to showcase skills they just do not have. In the immortal letters of my bass player, (while watching one of the several millions Sweet Child of Mine solo cover videos) "W.T.F.?"
Frusturated and suffering from slight nausea, I set out to discover exactly why my ears have to bleed. If you are a sub par soloist or even general player, at least one of these will most likely explain your situation. If not, then read it anyway and you might be able to help another starving artist.
Only Playing Other People's Music
Who doesn't want to put on a black "R.I.P. DIME" cap and bust out the "Walk" solo perfectly note for note while in front of their metalhead pals? A conservative, classical player, that's who. Trying to emulate their favorite artists restricts too many players from being able to explore themselves musically. The vast majority also don't understand the theory behind the solo, so if they were to perform it live they'd crash and burn if one note were to be missed.
Before picking up electric guitar and the whole rock and roll deal, I was a Jazz trumpet player. I got better at improvisation simply by playing a back track and attempting to solo over it. After warming up, that would be the largest part of my Jazz practicing... the same can be applied to guitar. Find a backing track and figure out what scales are appropriate for it. A huge part of getting better at soloing is learning first hand how to pull parts out of your lower cavity. It takes a little bit of patience and a whole lot of messing up to become "good."
Inability To Play Rhythm Guitar
Out of the garage band set-up, even moreso than bass, Rhythm guitar is the most misunderstood. Playing a few chords, that's it right? Naive players never gain the ability to learn the most important part of having a position like rhythm guitarist- rhythm. You can't randomly hit notes at varrying speeds and make it sound pleasing to the ear. Even Yngwie lands on the downbeat kiddies. If you can't play rhythm, you can not play guitar at all. The complex strumming patterns that an accomplished player knows are the basis for picking patterns and phrasing of solos.
Lacking Knowledge Of Theory
Theory is all the components that go into making music. It takes two lifetimes to master theory, but understanding a good amount of it is beneficial for even knowing which notes would sound in place and what play next. There's a heated debate between theorists and others that refuse to learn simply because they believe theory will ruin their free spirit. Individuals like Jimi Hendrix have natural ability that enables them to solo without the help of theory, but it's ironic that those who don't learn theory are the ones who don't have that natural ability. What are they gaining by trying to be successful with skills they don't possess?
Not Able To Apply Theory To Soloing
You know the modes, mindlessly practice scales until your fingers bleed and you've memorized your $40.00 theory book cover to cover, but when it comes down to it you can't even jam to a simple A-family Chord progression. When you try, all you can do is run up and down a scale and make a sad face at the audience. You're trapped inside the pentatonic box going "Help me!" Enough with the analogies and metaphors... a big thing musicians don't understand about theory is that it is meant to be a basis for music. Scales and boxes are meant to be extended upon. You should use the scales as tools, they shouldn't use you. When you switch your mentality up, you'll feel more free on the fretboard and less contained in terms of notes you are "allowed to play."
Shredding Distraction
Most players want to be able to shred. Shawn Lane, Steve Vai, MAB, etc. are viewed as guitar deities as a result of their speed. When watching a video of such a player for the first time, a lot of guitarists tend to get excited and get in way over their head. All that comes from trying to instantly play 21nps is frusturation and wasted practice time. Like any other style of playing, shredding takes a long time to get good at and a lot of dedication. In my eyes however, shredding produces the greatest amount of worst musicians. They gain a good amount of speed but that's been their only focus for who knows how long. On top of that, they end up cocky since they are able to play faster than the person that's been playing for 2 months. Guitarists that are just beginning to solo should not be aiming as high as material written by guitarists that have sold their souls to their instrument.
No Emotion
In all the theory mumbo-jumbo and the polictics involved in not sounding like someone else, soloists fail to place themselves emotionally into every note they play. In an unexplainable phenomenon, even the average non-musical person can tell whether there is a machine or human behind the guitar. How do you put emotion in your music? If you have to ask a question like that, you just don't get it. Heh.
The Whammy Bar
Sweet, you just shelled out enough to feed a small family to buy an axe with a Floyd Rose bridge so now you're ready to dive and squeal! This is what I call the "new toy" effect. When players get a guitar with a whammy bar for the first time it just seems to take them over. Much like guitar effects, a whammy bar should be something that is layered over your solo to give it a different sound and excentuate portions of your solo. A lot of guitarists take it so far that their solos are based solely around that pretty rod. My general rule is that you should be able to play your solos dry on an acoustic.
Failure To Return To The Basics
The basics! Nobody is beyond the basics of guitar. When young players start learning "advanced" techniques they tend to instantly jump into those during practice. In my opinion, the reason advanced techniques such as inner string sweeping are so difficult for players is because they never took time to fine tune and master their basics. Posture, muting, and proper fretting technique are the three most common reasons for this. When a technique is not learned properly, it can't be applied to soloing or even normal playing without it sounding bad. Vibrato and bending technique are a pair of the most misused basic guitar techniques. A tree can't grow unless its roots are planted firmly into the ground.
Having The Title "Lead Guitarist" Before Being Able To Solo
Uhg, personally this is the one I hate seeing the most. In bands with two guitar players, the better one or the one that thinks he's/she's better takes the title of lead. Usually bands that put so much emphasis on titles suck. Plain and simple. Their guitar players suck and they take the term "lead" in vain. They can solo since the "Smells Like Teen Spirit" solo is within their reach.
...
Yeah.
Little Exposure To Better Musicians
In a world with so many varrying music styles and players, there is so much that can be learned from other players. I highly recommend sitting in on some Jazz jam sessions, even if it's not your style. Some of the most accomplished, underappreciated guitarists aren't out selling DVDs. They're in a coffee house in lower east side of New York playing their hearts out. Actively jamming with people who are better or even close to your level with teach you so many things, including how to think on your feet for soloing. Don't be full of yourself to learn from others. Improv solos take balls and experience playing "live" to be above average at.
Soloing is what gets players high on stage. It's an almost orgasmic state that real musicians die for. A creative existance is more than most can hope to live. Don't limit yourself by being stupid. Soloing is the golden bullet in you music arsenal. I'm overdramatizing. Just have fun with it.
-NotAJock2Day
Learning was a lot easier for me because I've been learning violin for 7 years :-P I'm anything but an expert, so not even I rely on my own opinions on this to be right... 
