Interesting question, and it does present a dilemma. The problem with giving uber stuff is that it could easily impact the players satisfaction of continuing to play. Say you give out a hammer that kills anything in one shot, Pretty soon anything combat related becomes boring. The satisfaction of having to fight through a tough dungeon is suddenly gone. On the other hand, give out nothing and the player feels cheated.
Mid point in mods I tend to reward players with potions, decent lockpicks, something along those lines, maybe gold. If gold, I try to provide at least enough to pay for the "costs" of running quests. I do try to avoid anything that has lasting impact on the game and try to focus more what may be needed to help the player along during the mod.
The problem with gold is players usually have enough, or can find more than enough by simply taking what they find and selling it. In all honesty, most of the gold a player will get is from the chest that are scattered around with levelled gold. Giving it out is more a marker of completion so the player doesn't feel cheated. Usually by level 4 or 5, I'm done looting and only take what I can use.
End rewards are a bit different, and depend mostly on the mod for me. Smaller mods, I may give out an item of note, Maybe something the player shouldn't be seeing for a level or two above recommended playing level.
My larger mods Definately at least one item of note, but usually it's a title. Best example of that was Windfall, my Oblivion mod. I tried to present the player with moral dilemmas in several quests. End rewards varied depending on how they resolved the quests. Doing things a certain way ended with being named Count of Windfall and a castle at one end, being run out of town at the other. Of course that easier done in a mod with 50+ quest and eight different endings then a simple 2 - 3 quest mod. Korobal Island, a large Morrowind mod, I gave the player an income producing mine and living quaters.
For me, from the player end, I usually don't care about end rewards. If I have a house I may place it as a trophe. I care more about the mods itself. If an adventure, give me an interesting story line, let me feel like I can affect the outcome by my actions. If a dungeon crawl, give me some mental challenges with the combat, not just a hack and slash. If a quest or faction mod, give me some areas where I have to make choices and actually live with those decisions. Harder to do on the modders side, but if done well, the end rewards can be anything, the real satisfaction comes from playing the mod.
Sorry for the wall of text, but you asked about something I actually have an opinion on. And if you figure out the magic formula of what are good rewards, let me know please.
Ed