Without going into too much theory (theory is boring and everyone always disagrees about it and whines all frikkin' day anyway) here are some good tips for improving the subjective quality - the "good"ness - of your solos.
Learning to play fast shredding is useful, but just like getting a PHD in something, you run the risk of losing touch with your audience. Ever take a class from a PHD?! Same thing. You will be playing some Melodic minor mode with an altered ninth at 500 bpm, and the only one who cares will be you, or maybe some other wanker if you're lucky enough to get them out of their parent's basements long enough to come and see you.
So what does that mean? Well, just like in sports, or academics, knowledge is good, but it is ultimately limited. Truly good soloing combines knowledge - and surprisingly little in some cases - with inspiration. Most importantly, a good solo will communicate with your listener on a higher level. By that I mean you could play your song in Germany, Japan, Iraq - anywhere - and people will "get it." Look at Pantera, Metallica, Santana, Pink Floyd. All of them have accomplished this. The audience may not speak a word of English, but they "speak" art. A good solo absolutely must communicate on this level. Otherwise, who cares?
Speed is also vastly overrated. Speedy licks are like fancy college words. Cerrtain words, like "avunculocal," which simply means you reside with your maternal uncle, often fail to have any impact at all because people don't know what the **** they mean. A good speaker, like Martin Luther King, Jr., will pepper his speach with spicy words, but the audience, including children, will understand. Rather than filling your solo with lots of "fancy words," so to speak, try making a few spicy licks or fast runs work effectively for you. Make a fast lick be a crescendo to a passage, or use it to complete your improvisation. A solo full of fancy fingerwork will only impress one person - you. So make fancy fingerwork work for, and not against you. If you can't make it work for you, don't do it at all.
Often neglected are bends and slides. David Gilmour is not a great soloist because he plays esoteric jazz scales from Juliard music school (he never went to Juliard.) He is a great soloist because he communicates with his listener emotionally, and because he makes fantastic use of slides and bends. Case in point - "Comfortably Numb." Tell me you don't just about crap your pants or cry or have an out of body experience, or all three, every time you hear that. When that comes on and I am driving, I pull it over! I am a danger to myself and others when David Gilmour pours his soul onto the fretboard. Driving is bad!
Finally, think about the solo holistically, like you would an essay or some other written or spoken piece - remember, your guitar is "talking." When you write an essay, you have an intro, a statement of the problem, your stance on the issue, support for your stance, maybe a few examples of conflicting viewpoints and a conclusion. Likewise when you solo, it should have a beginning, an introduction to the main ideas, a climax and a conclusion.
Music is the highest form of communication. The Internet is like sticks and rocks to music. Cell phones are like cave paintings. Primitive, primitive, primitive! It's not the notes you play, it's why you play them. It's what you have to say, not the words themselves.