Apart from the cut of meat used, the other thing that affects the result and texture is the temp you use for the slow cooker. Meat is best at low temp for long cooking time.
If the temp is too high, fat present in the meat 'melts out ' into the liquor and will also affect the result, so trim offf any larger bits of fat too which should help.
Make sure you do use a good temp though as this kills any bacteria present - necessary to avoid food poisoning....
Searing the meat is to caramilse the naturally occuring sugars that aids the flavouring and will also remove a bit of the exces fat, but it isn't strictly necessary. (It does assist in cooking a trad. stew though, so definitely recommend if you just cook in the oven).
I would also brown mince before using in a slow cooker - and pour away the excess fat too from frying.
In using the slow cooker, if you make a veg and meat stew, put the veg at the bottom as they take longer like this to cook than meat.
And you say its a joint thats the problem - slow cookers actually work best with small dice rather than large full joints but so long as you have a good amount of stock / liquid it should be ok - try a gammon joint in cola, or lamb shanks in a wine and veg stock......