You can get decent battery life from the N900 so long as you don't run anything that stops the processor going into its low power sleep mode, you want to use powertop to look into this rather than top, but I won't help you with this, if you want to sort it out there are plenty of threads on battery life you can look at. Also it may be that your battery is worn out and you need a new one.

You mention some problem about permissions as root? You're not very clear on your exact problem, but it may be a corrupted MyDocs, in any case it won't hurt to follow these steps to fix any filesystem problems on MyDocs.

Stuff not showing in the media player, that is likely to be a tracker problem, run "tracker-processes -r" to delete the tracker databases and restart tracker, in about 5 minute (maybe longer if you have a lot of files) your media files should have been reindexed and will show up again.

But if you really aren't happy with your N900 then sell it and get something you will be happy with.