Jacob, you've received lots of good advice here, including reading up on passes at Seat 61, making a sample itinerary for the purpose of pricing your intra-European travel with and without a pass, and considering the marginal cost of/savings from choosing one pass over another.
I want to emphasize this last tip, which was from Laura. If it turns out that a decent number of your trips can be covered with a pass, going from a select (x days within 1 or 2 months) to a consecutive pass (such as 1 month), or to a longer consecutive pass (such as 2 or 3 months) provides a big increase in flexibility for a small increase in cost per day.
Yes, you do need reservations for many cross-border trains and for fast trains within many countries, passholder reservations can be expensive for high-profile services like Thalys, and some branded or private trains don't accept passes at all. Nevertheless, you will find, especially in western European countries, many trains for which reservations are optional, or not even possible. In pass-unfriendly France, a basic domestic reservation costs €10. In Germany, it's less than €5 (second-class).
Gaps in pass coverage for high-end trains have been widening for decades, but a more recent development is gaps in pass coverage for local trains. Regionalization and contracting-out mean that local trains on some lines are no longer operated by the national rail company. It's really important to confirm that the local operator either respects the tariff of the national rail company, or directly honors rail passes. Some short trips require a local transit ticket (whose price doesn't fluctuate like the price of a long-distance train ticket), and cannot be covered with a pass.
I encourage you to download Rail Europe's "Rail Planner" app and check the "Pass network only" box to find trains covered by the pass. The app also indicates where reservations are required. (Sadly, it doesn't reveal where reservations are possible but not required, let alone where they are recommended. The apps of the national rail companies do reveal reservation-optional trains and may reveal busy days and times when it's wise to reserve.)
Accessing Rail Europe via any of the "Buy" links on Rick's rail pages lets you price passholder reservations. Go to the shopping cart, start a booking, check "I have a rail pass", and indicate that you have a 2nd-class and "global" pass.