A lot of the disappointment comes down to what expectations were/hopes (there was a chance Democrats would outperform the polls, after all), the Senate, and losses in a few high profile races (Florida and Georgia foremost, but Texas Sen and Ohio Gov would count too).
Overall we're looking at:
- Yes, losses in the Senate, probably net of 2-3 gain for GOP
- But we gain House control, 30-39 seat swing to the Democrats, which is actually means it will probably come in just slightly below 538's model's prediction (so expectations were actually pretty much met in that regard)
- 7 governor seats flipped (with CT left to be called, possible GOP flip but Dem leads narrowly) (also this meant defeating Scott Walker and Kris Kobach)
- Several trifectas gained - IL, NY, CO, NV, NM, ME
- Obviously, many gains in state legislatures
- Anti-gerrymandering referenda passed in multiple states
- Automatic voter registration and similar voting reforms passed in multiple states
- Felon disenfranchisement largely ended in Florida, increasing eligible voters by 1.5 million
- Marijuana legalized in Michigan
- Medicaid expansion leads in multiple state referenda (and as a result of new governors, will also go ahead in KS and ME)
There are other things that went well.
Overall, pretty good results that will cause significant problems for Trump and enable some important changes, even if I really would've liked to see Abrams or Gillum win, or better results in the Senate.