As a Recruitment Manager, the main things that sound alarm bells for me in terms of reasons for leaving are things like 'disagreement with employer', 'too low pay' (you knew what it was when you accepted the job and does this mean you're entirely money motivated?) and things like that. Nothing wrong at all with saying seeking new challenges as long as you are happy for an interviewer to probe this at interview... after all, they won't want to offer you the job if they feel you'll be bored of it after 6 months and leave.