As much as I love the models there was always something that put me off Tyranids: the lack of personality. This extends further than them just not having names or even named characters in the traditional sense. Because Tyranids of whatever size or intelligence are birthed by the hive ships, perform the task they were made for and are then digested by those same hive ships its unlikely that (storyline-wise) your Hive Tyrant in one game represents the same Hive Tyrant in the next.
Essentially, they're the one army where you aren't playing the hero in your own story.
No matter how debased or evil or mad the leader of any other army might be they're still pursuing an agenda. Your Necron Lord wants to rebuild what he sees as the one true civilisation of the galaxy. Abaddon the Despoiler has a whole revenge drama going on even if his ultimate objective is to throw the galaxy into eternal damnation. The motivation might not be nice but it exists, there's a way that the character sees themselves as in the right, no one sees themselves as the villain of the piece, as the old writing maxim goes.
(Okay, maybe the Night Lords.)
In a way this very quality makes the Tyranids perfect for my purposes. I'll be playing a lot of club games with this army as it grows and trying to impose a storyline on that sort of environment is a nightmare. If I can just handwave fighting Tau, Space Marines, Eldar and so on and so on in rapid succession by saying its all different splinters of Hive Fleet Jormungandr all the better.
It'll also be interesting to see what sort of story my more regular opponents come up with surrounding my actions. My friend Tom (a dedicated Valhallan Guard player) and I have been trying to come up with a storyline for us to play through for some time and the classic horde vs. horde or Guard and 'Nids seems as good a story to tell as any.
And if I eventually find myself with a yen to write some personalised stories of the campaign I can always bring my Genestealer Cults into the mix.