Seqables are odd in Clojure. Some functions auto-`seq`, some don't.

seqables_are_odd.clj

(empty? (d/datoms db :aevt :user/id))
;;=> false

(not-empty (d/datoms db :aevt :user/id))
;;=> #<db$datoms$reify__3265 datomic.db$datoms$reify__3265@7939b07>

(seq (d/datoms db :aevt :user/id))
;;=> (#Datum{:e 17592186046081 :a 70 :v "abc" :tx 13194139534976 :added true} ...)

(first (d/datoms db :aevt :user/id))
;;=> #Datum{:e 17592186046081 :a 70 :v "abc" :tx 13194139534976 :added true}

(rest (d/datoms db :aevt :user/id))
;;=> (#Datum{:e 532163627844696 :a 70 :v "def" :tx 13194139534423 :added true} ...)

(list* (d/datoms db :aevt :user/id))
;;=> (#Datum{:e 17592186046081 :a 70 :v "abc" :tx 13194139534976 :added true} ...)

(map :e (d/datoms db :aevt :user/id))
;;=> (17592186046081 532163627844696 ...)

(count (d/datoms db :aevt :user/id)) ;; WTF?
;; UnsupportedOperationException count not supported on this type:
;; db$datoms$reify__3265  clojure.lang.RT.countFrom (RT.java:556)

(vec (d/datoms db :aevt :user/id)) ;; WTF?
;; RuntimeException Unable to convert: class datomic.db$datoms$reify__3265
;; to Object[]  clojure.lang.Util.runtimeException (Util.java:219)