Abstract:
Taking the bass into territory that used to be reserved for guitarists and other treble instruments is admirable, but you have to have a strategy if the results are to be anything other than monotony and pain for your audience.

Body:
If you've ever spent any time culling through bass solos on YouTube, you know what dreck lives there. Every fifteen-year-old with access to a video camera seems willing to share his or her first painful experiments with bass as a solo instrument. Those of us who learned to play before the advent of the web should count our lucky stars. But as bad as many of these attempts are, they can be instructive to anyone interested in the bass as a solo instrument. A recent late-night journey through YouTube inspired me to categorize a few of the bad bass solo examples out there and offer some thoughts on a better approach.

The flury of notes

The most common bad bass solo technique is the flurry of notes--generally in only one rhythmic grouping. Bassists with sufficient speed often rely on it to the exclusion of any other consideration. Consider this first example, from metal band Angra:

First off, a few caveats. This is a bit of live concert footage and shows a fairly typical metal staple: the guitar battle. In this case, it's a guitar-and-bass battle. For decades, arena rock shows have featured segments like these, and there's no reason to assume the same sort of quality playing here that you would expect on an album track. Most players, in this playing situation, just pull a throw down a bunch of fast runs to excite the crowd. And the players here do the same. It's also obvious that both the guitarist and the bassist have considerable skill on their instruments. The sort of speed both of them manage is not easily achieved or maintained.
But, that said, notice how little it matters here. What we have from the bassist is a flurry of notes. There are no pauses, dynamics, or repetitions to emphasize particular notes out of the sequence. In fact, almost the entire solo is played as if it were one phrase. There is no groove to speak of. Listening to this performance, I'm astounded by the speed, but that's all that is on display. And, because of this lack of any other musical element, the solo is entirely forgettable. None of it is likely to stick with you after the video is over. If the point of the solo were merely to display the bassists admirable three-finger technique, bravo. But if the point were to put forth a musical statement, beyond the demonstration of near flawless technique, it is a failure.
Examples of this sort of playing abound on YouTube, though generally from players with far less talent. Consider this, rather typical, example:

Here we see the same desire to impress purely with speed, but the result is far less effective because the player lacks the physical ability to maintain the note bursts for very long. On top of that, the execution of the notes is not consistently clean. If you are going to attempt to rely entirely on technique, you have to have that technique down pat. Angra's bassist has it; this poor kid doesn't. Give him a few years.
One more example should be enough:

Here, the note choices themselves are a little more thought out--as they're largely based on arpeggios--but it still sounds like something you'd play to warm up, rather than something you'd play as a solo. And the same reliance on speed and lack of rhythmic diversity and dynamics is on clear display.

Slap me silly

The same reliance on speed over grove (or, in fact, over any other concern) is easy to find among slappers. The first 24 seconds of this next example is pure speed. The line, at this tempo, makes no sense at all:

Something happens at 24 seconds in: the bassist stops relying purely on speed and lays down an effective groove. He even pulls some impressive licks without sacrificing the groove he has established. But, as he continues, his willingness to loose the groove in favor of some fancy lick takes over (at about the 35 second mark).
But it could be worse. It could just be a bunch of notes played as fast as possible, without any consideration for groove, like this:

What would be better? Well, for one, some repetition and some sense of groove. That doesn't mean speed is out. It just means that speed has to exist in some context. Take this example. It's not the best playing ever, but it far better than the previous two examples:

Notice that the bassist begins with a strong rhythmic pulse. On top of that pulse, he establishes a nice rhythmic figure with plenty of rests as well as notes. At 16 seconds in, he pulls a very tasty speed lick, but it's still grounded in repetition. You can still follow what's happening. It gets a little cloudy toward the end of the fast section (22-31). This is partly an effect caused by the cheap mics on most consumer video cameras, which amplify the treble and sacrifice the fundamental of the note. And the bassist gets a little ahead of the beat. But, he pulls it out and returns to the groove.

What can we learn from these examples?

• Relying on speed alone to support a solo is, at best, a highly problematic strategy. First off, you have to have amazing technique. Bassists aren't normally called upon to play blinding fast runs. So, when they do, their lack of practice at such tempos shows. But even bassists who can pull off those sorts of tempos aren't impressive if they rely on speed alone.
• Rhythmic diversity helps a great deal. If you can establish a groove, then shifting to a faster tempo can be impressive--especially if you don't loose the groove and can return to it at will.
• Dynamics help. Not only should you consider notes of various lengths, you should also consider notes of various volumes. The bass has an impressive dynamic range. You can use it to your advantage. If every note is the same volume, none will stick out from the others and the chance that your playing will sound flat increases.