I've watched Street since he's came up, and I was at the game on wed and saw it on TV last night. Heres the thing, and this is a bit of a carry-over from last year. Street struggles against leftys. All 3 of the HRs were against lefty's.
He added a changeup in 06, which was very effective at first(even striking Bonds out with it to end a game), but lefty's are laying off of it now, and he doesn't really throw many sliders(by far his best pitch) to leftys, for obvious reasons.
The central issue though is the velocity on his fastball. When Huston first came up, his 2 seam fastball had good sink and its velocity was in the low-mid 90s(topping out at 96 from time to time). This surprised many, because he was not throwing this hard at Texas and was the main reason he was not drafted earlier in the first round.
Huston missed a good amount of games last year with "ulnar irritation" in his throwing arm. He came back towards the end of last year, but was not throwing quite as hard(typically being more around 90), which he was still effective with, but not as dominate.
Last night he hit 89 on nearly every fastball I saw him throw, never breaking 90. He can live and be decent(but not an elite closer) with that velocity. But what is happening now is that leftys are sitting on that pitch, and laying off his changeup. To be more effective against leftys Street is going to have to find a bit more velocity or perhaps some other effective offering against them besides just his fastball/change combo.