simply like
my bluetooth is not starting
my hdmi is not working
Etc bla bla...
by posting lamely, U r not helping the developer u r just spamming his thread
to really get help, make sure u have a logcat of your problem
What is logcat?
Logcat is the command to view the internal logs of the Android system. Viewing logs is often the best way to diagnose a problem, and is required for many issues. This way you'll find out what apps are doing in the background without you noticing.
Advantages of Logcat
Debugging
You can see what processes are running, if a process is running after a certain interval of time, it will eat battery. So you can also find out what is draining your battery.
What developers need is a logcat To know what is the issue
i will tell u how to make it on a phone itself
1) download an app called ALOGCAT
2) install app and open
3) u will find random text coming there (its not random literally

leave it in background by pressing homekey
4) this step is specific
for eg. if are having trouble in switching on wifi , try to swtich it on or do fiddle wid anything u have .. just play around with your cell, run the app u think is buggy or whatever

once done
5) press home button for long, select alogcat app,
6) click on options hardware button (right one

logcat will be saved to sdcard/alogcat folder in the form of txt files.
7) now either u can directly upload the logcat.txt while telling about your problem
or
u can copy the contents of that txt file and paste on www.pastebin.com
after pasting u will get a link something like pastebin.com/******
just copy link and paste along wid ur problem
8) now next time u have a problem make sure u attach a logcat along with it
this method is not useful if u r not able to boot the phone
for that u need to have a pc and android sdk installed along with the drivers
if u have everything installed then
Open the 'run' dialog by pressing the 'Windows' + 'r' buttons on your keyboard (minimize the browser & other non-windows programs)
cmd (this will open a DOS prompt)
cd c:\Android\tools (go to the directory where you extracted the SDK)
adb shell
Now you'll see just a '$'
logcat
You can now just press the power button on your phone to see what happens. It displays everything the device is doing.
This is my first post regarding logcat , so i m sorry for any discrepancy (if there, notify me plz)
P.S - logcat is also useful in case if u r having some trouble wid stock roms

thanks @El_Dark for extending the op.
When you guys are experimenting on a custom rom (i.e. nightlies, betas, RC's, "just cooked", etc.), it's very likely that your phone might not boot correctly, fear not and instead, try using ADB

Now, to do the said logcat, open a command window and do:
C:\Users\Dark> cd \<Name of your Android SDK folder>\sdk\platform-tools [Hit ENTER!]
C:\<Android-SDK folder>\sdk\plarform-tools> adb.exe devices (just to make sure your computer detected your phone lol)
C:\<Android-SDK folder>\sdk\plarform-tools> adb.exe logcat -v long > somefile.txt (the "-v long" part creates a nicely formatted file

That's it!! Remember to zip the file if it's too big

Another tip: Some phones are very catchy when under heavy issues (like mine

