Still presentation and writing. Old shows are full of movement and progress. There is certainly reuse in animation but other than that it keeps moving most of the time, the plot progresses pretty fast, the dialogues are short and concise and the characters get a real and proper development.
Whereas nowadays most anime don't have any motion at all, plot progression is slow and dialogues, albeit longer, lack any actual depth and sound like written by 3-year olds.
If you don't know what I'm talking about, watch Turn A Gundam and then watch SEED/Destiny/00. Turn A, albeit being a quite young show has all of those things series in the '80s had, it's goes back in every aspect, from presentation to writing, while the new century stuff suffers from everything tv animation is plagued by in the last decade.