Time Travel, Baby
The concept of time travel is beyond fascinating, but it isn't novel. Many shows have dabbled with it and oftentimes, unsuccessfully. However, Timeless' formula works because they follow their own rules and it actually advances the plot.
Time travel can get really messy, for obvious reasons, so it's important you are familiar with how it all works: they aren't able to go back a time where they already exist because a run-in with yourself is awkward and life-threatening. You cannot go back to the same moment in history twice which adds a level of permanency to their missions. The butterfly effect is also why Lucy is so adamant about preserving history; one wrong move can erase people, cure them, bring them back to life, or set our society back thousands of years.
There are always exceptions to the rule, however, and the writers have figured out how to bend their own rules without breaking them. You'll just have to watch to see what I mean.
The Writing Is Unexpected
Sometimes, you'll think that you know the outcome of a storyline, but the writers are so clever, they pan out twists that you never really see coming. These writers just have a way with cliffhangers that again, aren't done for the sake of doing them but actually advance the storyline. They also have figured out the perfect formula for exploring real human emotions, relationships, and insecurities that fans can relate to even if they are confined to their own time period.
Timeless excels in the diversity department. Out of the whole cast, there are only three caucasian actors, one of whom is a vulnerable man and the other, a female historian. Jiya and Agent Christopher are two strong female characters; the former is in an interracial relationship and the latter, in a same-sex marriage. Rufus and Connor are both tech-savvy and successful black men who are forced to reinvent themselves. Timeless has really made sure that they hold up a mirror to today's society and showcase how people from all walks of life bring different experiences and necessary lessons to the table.
The Big Bad
How I got through this whole thing without once mentioning Rittenhouse is beyond me. In the series, they mention the big bad roughly 100 times in every episode and with good reason: Rittenhouse is a formidable threat to the very fabric of America. They are also the reason for all of the trio's missions. Flynn first warns us about the very-real-threat that they pose and soon, we learn that they have control over Connor Mason.
Though their agenda isn't always 100% clear, they have ties to powerful historical figures, access to various time periods, and a deep need to eliminate democracy and take over the world. Did I mention there are no limits to how far they will go to get what they want? Yep, I know all this conspiracy theory is what really got you hooked on the series!