Not enforcing a follow on when 398 ahead on the first innings.
Mental (and physical) degredation?
There's only a few days between tests - if follow-on enforced, our bowlers could be on-field for two full days - very hard to backup from. Watson, Harris pretty fragile at best of times. Johnson was flying, but the rest were ineffectual, bowling to the tail, with the opposition on the ropes. If Johnson fatigued (from what I saw was already happening), the chance was England could be 0-100 at stumps, and whilst not back in this game, mentally back on par with our bowlers.
By not following and likely delaying a declaration we essentially avoid the possibility of losing. With 180 overs to go, and reasonable weather, i wouldn't mind seeing us bat until just before tea - as much to unsettle the openers "preparing to bat" psyche, as to give their bowlers the maximum time in the field.