Some questions I'd ask, having been on both sides of the table:
1. For approximately how many start-ups have you been lead counsel for (a) incorporation / LLC formation, (b) angel funding, (c) Series A-[n], (d) IPO? How many in the last two or three years?
2. References? Any clients we've heard of? (The lawyer might not be able to tell you s/he represents Client X if it isn't a matter of public record.)
3. Any litigation experience? (Other things being equal, give me a lawyer who has tried cases - it changes your perspective quite a bit.)
4. Any in-house experience? (In-house lawyers often get some valuable lessons about business life that you just don't get in a law firm.)
5. [EDIT:] Your billing rate? Likely total cost of this matter? Do you do any fixed-fee work for specific types of project?
6. Whom do you use as backup? (Even in a law firm there can be de facto sole practitioners.)
7. Who will actually be doing my work, what kind of background does that person have, and how much work do you do with that person? (An associate who gets a lot of work from Partner X might be just fine, because (i) s/he will have an incentive to keep Partner X happy so the referral work keeps coming, (ii) Partner X will probably talk to the associate a lot, meaning Partner X can ask how your matter is going and may well do a lot of day-to-day mentoring, and (iii) chances are the associate can get ahold of Partner X whenever needed.)
8. How often do you bill? (Some firms bill quarterly, which might be fine, but most companies prefer monthly so as not to get unpleasant surprises.)
9. Is it feasible for us to establish a fee level where you'll call me if you're going over that level? (This is a tall order for many firms' billing systems, but it can't hurt to ask.)
10. [EDIT:] Is your usual inclination (a) to try to negotiate the maximum possible concessions from the other side of a deal, or (b) to accept a good-enough deal quickly and move on? (My experience has been that option b is usually preferable, especially if you're negotiating with someone with whom you're likely to do business again.)
11. [EDIT:] Will you call me with updates, or should I be calling you? (Good lawyers are busy and might not always take the time to provide real-time updates.)
12. [EDIT:] Do you carry malpractice insurance? (Some sole practitioners don't.)