First of all I've removed the annoying and power eating boot video by changing the file /etc/hildon-welcome.d/default.conf to the following:
[hildon-welcome] # filename=Hands-v32-h264.avi
You see I've just commented the filename-line. This won't show the video anymore but there will be a short moment 1-2 seconds the screen will be black.
Maybe there is a possibility to remove this, too.
I'll look into it later.
The second annoyance for me was the terminal.
WTF do they use ash as default although bash is installed with completion??
I don't understand it but I've changed it for me.
Just add 'bash' to /etc/profile and your terminal will start with bash.
Unfortunately this doesn't change the default shell for root but I'm mostly using the normal user so it doesn't disturb me much.
A good How-To for flashing an N900 can be found on Maemo.org.
So I won't describe it here but I've encountered an issue concerning the available OS binaries of maemo 5.
Some programs like powertop are shipped with the first version (1.2009) only.
So if you don't want to patch until the newest version and can live without these missing programs you can use the second version for flashing but if you want to have all programs you should use the first version and then patch using the app-manager until the newest version.