i) Multiple sounds.
ii) more than 70 keys, yet provides ease of mobility.
iii) Headphone jack , jack-in & jack out.
iv) touch sensitive or weighted keys.
v) Good organ sounds & horns/ brass section.
vi) USB connectivity & user tone extendability.
vii) Storage banks & sequencer (although my board lacks a bit there
)
viii) Effects.
ix) Pitch bend & modulation features.
x) reliability.
xi) Affordability ,compared to boards with similar features.
xii) For my 1st board, I also had to ensure that it had on board speakers because I don't have an amp & a good set of speakers yet.
That's what I assessed before opting for the Casio wk3800..