In reply to Blizzard:
Interesting. I definitely feel an increased sense of duty if I'm climbing with someone who I know will find seconding the route difficult. I'd never go out of the way to give my second a hard time, but there have been occasions when my second wants to push their grade a bit.
If I'm with my regular partner who climbs about the same grade as me, I would build a belay that I know I would be comfortable seconding on. Equally, when seconding, I accept that some routes don't have fantastic belays and would expect my partner to tell me that the belay's not very good and I would make a judgement.
If I'm with someone who's pushing their grade when seconding me, I would try and build a totally bomber belay with a redundant piece in it and If I couldn't I would tell my second what I've made the belay from and what the moves are like and let them decide, I'd also let them know if I think it's a bad idea for them to attempt the second.
Obviously the above applies to single pitch cragging only.