Can I use Word of Seizing on a planeswalker?
Yes, you can target any permanent including a planeswalker. You're right that impossible parts of an effect are ignored - in the case of this spell an example is that you can still target a permanent that's already untapped. Untapping something that's already untapped is impossible, so you just don't and move on with the rest of the effect. 

However, the part about gaining haste isn't actually an example of an illegal or impossible effect: the planeswalker, or whatever else you steal, will get haste. Most of the time this will be irrelevant (haste doesn't do anything on a non-creature), but if the permanent later becomes a creature it will matter. So if you steal my Gideon, Ally of Zendikar and then use its +1 to turn it into a creature, you'll be glad this spell gave it haste, because otherwise you couldn't attack with it.