See one example of a multibuild I made in the other thread.
Versatility and Synergy are the key advantages - could be one or both of them. You are aiming to get something out of the second class that will complement your primary class in some way. i.e rogue likes the warlock eyebite power since it will allow him to go invisible against one opponent and then stab him, this is also a ranged power which is in short supply for the rogue - this is synergy. A warlord might like to have an area blast wizard powers, just so he can blow up minions (this might be especially useful if the group has no wizard).
Lots of things that affect multiclassing and what you can do:
- Choose your primary class based on class abilities (not powers) as the second class grants you nearly none.
- Check if implements will cause you trouble - i.e. wizard going into ranger may find out that he cannot hold his implement out and still shoot a bow or use two weapons. A Warlock/Rogue however has the ever useful pact blade which is perfect for warlock implement and rogue weapon. Powers may become gimped at high levels for this reason. If you have no access to an implement, i.e. ranger wizard, you might find out that the powers are becoming less and less useful since the magical bonus of the implement will be needed at higher levels to hit anything with the wizard powers. At low levels you will not notice this at all.
- Check for MAD. Many multiclass combinations will require too broad set of ability scores to be really good at any one thing. I would not want to have more than 2 really high scores and even that is a burden. The new defense pairing of stats is also one thing to watch out for. You are somewhat gimped if you need to have a high stat pair that also is used for one defense. i.e You are a rogue wizard, you need high int and high dex. This leads to “wasting” ability bonus on your reflex and AC defense, since both add to reflex and AC. Rogue and Warlock are much better, as you use dex and cha which both are used for different defense - also because cha is useful to rogue and is used for warlock powers (dex is no use to a warlock class abilities, so Rogue/Warlock is better than Warlock/Rogue).
These are game mechanical advantages/disadvantages, obviously the character roleplayed well can be fun even as a bit of a suboptimal build.