The next round is just for the play off teams, teams that were positioned from 8th to 1st will be relaxing. For Lille and Aston Villa, though they had easy teams as you said, but they really tried in beating them and getting to the position at which they found themselves now. It likely that the Play offs will start in two weeks time.
You are right, teams like Lille and Aston Villa have fought quite well. This is a good challenge for them to develop better in the Champions League. But remember this is a competition that is quite stressful for any team to bring out all their abilities. And the teams in the play-offs do not mean weak teams, in fact we can say that we will see one of them in the final. For now, 8 teams can take a breath before going through a much tougher match.