Nyong'o and Waltz were pretty expected winners in 2014 and 2013 respectively. They had both won a lot of the precursory awards. I'd say it's pretty rare for upsets for Oscars in the last 5 years or so in the acting categories.
The only really surprising Oscar wins I can think of recently are Adrian Brody for "The Pianist," Marcia Gay Harden for "Pollock," Ang Lee winning director for "Life of Pi," otherwise the races didn't have really unexpected winners (this is off the top of my head).
I didn't think that Brody's win was an upset. He was great in The Pianist, and had received good reviews and some awards. I didn't think that Roman Polanski's past would impact his Oscar chances. He was against some tough competition, but he was sublime in the role of the Pianist. I can't ever forget his soulful eyes, which expressed so much without many words.
I didn't feel that Ang Lee's win for the Life of Pi was an upset either. Lee is a well known director and he was great, though someone else could have won in his place.
I had never heard of Marcia Gay Harden and I didn't see that movie until after the Oscars, so her win was a surprise to me.
I thought Denzel Washington (Training Day) over Russell Crowe (A beautiful mind) was an upset. Also Juliet Binoch (sp?) winning over Lauren Bacall (sp?) for the English Patient was a surprise.
This year, Mark Ryland had won a few awards early in the season, but lately Sylvester Stallone was winning them, so Ryland's Oscar seemed like an upset (he deserved the win, though I had not seen 'Creed' so I can't really compare the two).