Both has :
- Comedy and Romance
-Unique funny characters
-Boku wa Tomodachi is more ecchi
-Nisekoi has more romance.
The school set-up is apparent in both anime, not to mention both are harem anime. The main protagonists(Kodaka for Haganai and Raku for nisekoi) have their own harem in which there are two main female leads. The rom-com in both are very entertaining and each character has their own special trait that stands-out from others. Both anmie also put a slight focus on character relationships while weaving in the casual fan-service.
main guy has a childhood friend he doesn't remember who (kinda)
main guy is getting a forced marriage from parents to another girl
main guy kinda has troubles having friends (Nisekoi not much)
blonde haired and black haired girls for main characters
Sakunai (Haganai) is more bold..er, I mean more revealing (rated R+)
Other story threads
Plot, characters alike, similar theme, with equal genders
Both are very similar in plot, trying to get a single guy to fall for them. I can't be the only person who thought that Sena Kashiwazaki had similarities to Chitoge Kirisaki.
-The main characters of both anime had a childhood friend 10 years earlier, but they were separated and never saw eachother again. No matter how hard they try, the main character can't remember the childhood friend's name, amongst other things.
-Both anime are Harem/Romance/Comedy, but Boku wa Tomodachi ga Sukunai has more ecchi and fanservice, while Nisekoi focuses more on romance.
-Both anime deal with social issues such as making friends, and the slow uncovering of the characters' past is a central plotpoint.
Essentially, the whole anime, you'll try to ship the main character with one of the girls. They are very heart-warming comic romances involving past, love, perfect school life, and even more love.
-Both series sport a harem with multiple love interests for our main protagonist.
-Both series revolve around humour that is instigated through awkward romantic situations.
Haganai is a series near and dear to me for no particular reason, and Nisekoi is the closest to it I could think of. They both go along with the harem/harem-esque genre, but in Haganai the Generic Dense Harem Anime Protagonist™ isn't actually a Generic Dense Harem Anime Protagonist™. Talk about originality, amirite
It actually leans towards slice of life too, and has a healthy portion of lolis. 10/10