Well, I could say that the lack of any really good ads for any Windows Phone (the phones themselves, not Windows Phone itself) is a small problem.
Of course, word of mouth is always the best form of advertising (I got friends to wait for the Lumia 620 whenit comes out in the Philippines), but would it matter if only a few people know about it.

A marketing tagline would be something like this : "The Amazing Everyday", or "This Is Lumia", both from Nokia('s Marketing Partners)

Well, here's an example from me:
For the Nokia Lumia 920:
"Smile For The PureView"
Or the Lumia 620/820:
"Never Worry about the Back of Your Phone ever again"
Or the HTC Windows Phone 8X, in Blue:
"TARDIS Phone, anyone?"

Well, what do you think?