IMO, Lasith Malinga relies greatly on his shock value, especially in Test cricket. That's why he was very impressive early on - neither the Australians nor the New Zealanders could play him all that effectively because the angle he bowled from was so unorthodox. Then again, if ever a team has been renowned for unorthodoxy (whether it be in the type of bowlers they produce or just in overall tactics), it is Sri Lanka.
Naturally, he has become progessively less effective as he's bowled more, as batsmen get used to his angle (tellingly, he is also at his best during the first test in a series). He does have a stunning yorker, a sharp bouncer, a clever slower ball and can generate some reverse swing, but as his pace has dropped with the advent of injury, his strike power has waned. Unfortunately, he is meant to be an all-out strike bowler. IMO, he's not accurate enough to perform a holding role.
In the ODI's and T20's, he is a very effective bowler, because of the weapons that I just mentioned. He is probably one of the most effective death bowlers going around when fit (witness that 4-in-4 spell against SA, for instance).