I'm interested in watching how four big things unfold.
First, will the supporters of the candidates who lost the presidential primaries still show up in November to support their parties' candidates? There seems to be a lot of division among the supporters of the candidates in both parties, with Cruz, Kasich, and Sanders supporters seemingly very reluctant to get behind the presumptive nominees, for various reasons. How quickly will they fall back in line, if they do, and will they show up in November?
Second, how much will the impression that Trump can't win the general election actually lessen the sense of urgency felt by potential Democrat voters? If they stay home because they think the election is a foregone conclusion, while Trump supporters continue to turn out in high numbers, that could make things interesting up and down the ballot. The Clinton campaign and DNC will likely have to work hard to strike a balance between painting Clinton as the "obviously" better choice while also reminding voters how important it is to actually show up to the polls.
Third, on a related note, will voters on both sides, especially those who feel disaffected after the way the primaries played out, remember or recognize the importance of the Congressional elections, and what kind of a job will the parties do of mobilizing voters down the ballot? Over the last decade, we've seen how incredibly important party control of the House and Senate can be for the passage (or blocking) of a president's program, but that is often lost on voters in our president-centric election conversations.
Finally, will some inevitable event or twist throw off one of the campaigns and reshape the race? We still have six months until election day, and history shows us that economic shifts, national security crises, and candidate revelations and gaffes can happen in that amount of time. When elections are reliant on the small groups of voters in between the two parties, these things can potentially affect our election outcomes.