Nottingham really isn't as violent as people make out. As long as you don't wander into really bad areas, which you'd have no need to, and don't go looking for trouble you would have no problem.
As for the campus, it might look a bit green at the moment because not many flowers flower at this time of year, but in Spring/Summer it's beautiful, especially around the lake, with the ducklings and cygnets and goslings too. I don't know why anyone would hate the campus if they've seen all of it, it was by far the best I saw when I was going to open days. Maybe you only saw the science half?
As for accommodation, some halls are better than others, as will be the case in most universities. I was in Florence Boot and it was the best I had seen - clean, decent sized rooms and a generally nice feel to the place. The new bit of Broadgate is also amazing but there is an older bit which is still fine, but not so great.
Nottingham itself is a fab place to go to uni. The campus is close enough that town is only ten mins away on the bus, but the green campus makes it still feel like you're in the countryside sometimes. Once in town there's a huge mix of shops, bars and clubs to suit everyone but it's still a small enough city centre to not feel overwhelming and after a few visits you'll know your way around easily.
And why are people put off by walking? I'd rather walk the way from halls to dept buildings, either across the Downs or up from Broadgate, than through city streets. It's good exercise, and really doesn't take that long. Remember you won't be walking all day, every day like you do on open days.