1. I think the opportunities are limitless (or, rather, only limited by the individual virtual assistant - how far they're willing to take their knowledge, how much effort they put into learning new skills, or updating existing skill sets, etc.)
Concerns, in my opinion, are the same as for any small business - cash flow; finding, attracting and contracting new clients; long term growth; etc.
2. I don't know how to answer this question - I don't work on retainer so some clients need more hours consistently, some need less, most of my clients' needs fluctuate to vast degrees.
As far as trends in their spending - they are spending more as a general rule but that's partially because we are offering more premium services, at premium prices and taking on larger projects / higher-end clients.
3. Price, price and availability. We're fairly expensive, and we're very busy. But I always tell PCs up front that we're not the least expensive provider and they can find (much) cheaper services. We are, however, the best.

I no longer take on clients who are obviously price shopping though.