As Machinima becomes more popular, the tools and options available increase, whether it be because of the use of a mod or actually added in by the Developers. Halo 3 is one of the most notable developer additions (with the Replay Feature and Stage alterations), and Brawl seems to be becoming one of the most Game Mod-based, from stages to movements.
The jump in quality from Garry's Mod to Source Filmmaker in terms of animation is pretty much day and night. Barring difference in tools, any animator who works hard and frequently enough on their craft will improve.
Speaking of Garry's Mod, Gmod Idiot Box has also improved in animation quality over time as DasBoSchitt started to incorporate stop-motion animation into his Gmod videos.
The art style of Neopets became more polished and refined over the years, giving them an overall "cuter" appearance and more dynamic poses. One oddball change is Bruce, which was originally represented by a picture of Bruce Forsyth (then a cartoon caricature of the same before entering its current penguin design). The color also improved greatly, going from generic gradients and flat colors to actual shading.
In Tokyo Magic Star, first the art looked rather simplistic and chibi-like, with lots of wobbling lines, but as the series continued it got more complicated and realistic, with straighter lines.
Popular video series ASDF Movie has probably one of the easiest to notice, even by comparing the first installment to the second.
The Awesome Series by Egoraptor. Go compare "Metal Gear Awesome" to, say, "Pokéawesome". Keep in mind Arin Hanson is completely self-taught.
AtheisticYouTuberDarkMatter2525's animations have undergone this. At first, his characters were drawn with black dots for eyes and squiggly black lines for mouths that moved when the characters spoke. Nowadays, they have regular, white and colored eyes and proper mouths with teeth and everything. The artwork in general is much more polished and refined, wih much more details and better shading compared to how it was in the early videos.
You. Are you a writer or an artist? Go compare the earliest work on your Tumblr, dA, Sheezy Art, FA, etc. to the most recent thing you've drawn or written. The more you study and work at your craft, the better you will be.