unfortunately smart playlist + random play + Up Next is not same as iTunes DJ.
iTunes DJ was not a smart playlist, it's a dynamically generated playlist that can use other playlists as the source. Sure you could just go to your music library and shuffle it to play random songs but that's not same as iTunes DJ.
The biggest difference for me is that iTunes DJ will maintain current list of songs even if you make any changes to the songs. For example, let's say I have a smart playlist that contains all the songs rated 4 star or higher and while listening to the playlist, I decided I don't like the song anymore and want to lower the rating down to 3 stars, which would remove it from the smart playlist. When I was using iTunes DJ, I can change the rating, or any other attribute, and the song will remain playing. If I'm just listening to the playlist, iTunes will stop playing music. It won't move on to the next song, and if you hit play, it will play another random song.
Up Next is a nice addition but it's not a replacement for iTunes DJ. iTunes DJ + smart playlist was a beautiful thing for me.... I'm guessing there weren't that many people who used it like that .. but still this is a big loss for me.