Ideally, you want a balance of items available in your chest loot. The vast majority should be things to advance early game, then a smattering of items to ease mid game, and a few rare drops to make adventuring worthwhile into late game. Overall, the experience of exploring and conquering should be rewarding at any stage of the game.
Items such as Ender Pearls are almost always welcome, especially early on. Enderlilly Seeds (if XU is included) are even better, as they provide a renewable source, but take enough time to grow that they don't immediately quell the need for the Pearls altogether.
Other options include items needed for routine crafting. This includes ingots of iron and gold, Redstone, and the like, but could be extended to include items such as TE Machine Frames (of any grade), Pistons, BC Chipsets, and the like. Also, items like Itemducts, which are relatively cheap, but always useful are nice (particularly if they can be upgraded into something more powerful later, or if picked up later game).
Something else to consider is early access to materials. Being able to obtain Glowstone Blocks and Nether Quartz before you have Nether access is a huge advantage, but since those materials are used in later game recipies, it doesn't feel cheap when you get them after access is obtained. The same principle applies to rewards from other dimensions (Twilight Forest comes to mind, but others exist). This works better if the loot helps gain access to these dimensions (Obsidian is welcome through mid game most worlds), since that opens the player to more areas to explore.
One last thing for now, consider hard where the loot is found. Having Glowstone Blocks be found in a Nether Fortress isn't worth a damn, but having it be found in a Spawner Dungeon is a great boost. The same applies to and tools and armor in your loot tables. Finding Leather Armor in a Spawner Dungeon is OK, as the player is likely to find one relatively early while still under equipped. The same reward found in a particularly hard Recurrent Complex structure feels cheap, especially if the mobs you're fighting are wearing better than that. Weapons and tools are the same, although occasionally an enchanted iron tool can be as useful as a plain diamond tool (hell, an Unbreaking III Stone Hoe will last some people an entire play-through).