Similar conversation(s) have been had in detail,go to hapkido forum,and if you want you may release this same thread in hapkido or/and aikido forums as an example.
You might also want to study aikido theory on net with hapkido theory and go to aikidojournal.com and ask some questions.
To make a long story very short and give some hopefully satisfying answers:
They both come from daito-ryu aikijujutsu,but the one who says there is no more to that on both arts is rather foolish (both arts have had influence from other arts too)
Both use circular motion and non-resistance as their basic principles (considering that aikido uses bigger circles,while hapkido does smaller ones and can be more crippling)
Both arts can be translated as "way of harmony" in both korean and japanese and can be considered sister arts (there are huge differences though)
Both are internal arts,generally.
You would probably like it in terms of technique,so let´s rock:
Aikido:jointlocks,pinning,throws,striking (striking-atemi in aikido is usually for distraction purposes to follow with grappling,striking or kicking has been lost in many schools)
weapon defenses,defenses against multiple opponents,some weapon work (varying between schools) including use of stick.
Techniques done in standing,and sometimes in kneeling position.
Hapkido:everything above with technically much more,all basical areas of fighting included such as:groundwork,various strikes and kicks,pressure points,more weapon work in defensive and offensive manner,takedowns,chokes etc.
Here´s something to think of,hope it helped.
:asian: