Ending is kinda fail, yup.
Just remember the game is really meant to be played in playthrough 2. Once you've turned in that final quest you'll have the option to play in PT2, which is generally more challenging throughout & will take you to level 58 (assuming you've got the latest patch). With Knoxx you can max @ 69.
Once you finish the game in PT2, the entire game scales so everything is your level for the main game, & all the DLC which adds more replayability as then everywhere is a challenge.
Make sure you grab the DLC; they add a lot of gameplay, especially Knoxx. Most of us still playing the game spend a lot of time there.
If you like finding good loot, Knoxx has the infamous Crawmerax you can farm for awesome stuff, & the Armory as well (lots of chest

)
Moxxi is all arena battles, but also gives you the option for additional bank space for items. Also you get a skill point for each playthrough of the three short arena battles (total = 2)
Zombie Island has a really nice atmosphere, & it's always fun headshotting zombies
The Clap is probably the weakest of the DLCs; feels rehashed & lacking so i wouldn't really recommend it, though you do get a skill tree point for each playthrough (two total).
Sort of on-topic, I beat Zombie Island and then tried Moxxi's Underdome but it's kind of annoying. Do you have to beat each 'arena' challenge in one sitting? I've only played it 2 or 3 times but it seems like it never saves my "wave" progress.
Yes, you need to do the 5x5 waves all at once.
If you think that's annoying lol, imagine doing the 20 waves...