Just to recap my bigger reasons why:
My worries over the physical keyboard ended up being a deal breaker. Given my Centro's performance I fully expect the Focus to last more than my two-year obligation (which is more than I can say for the normal cheapy, "free with contract" phones I used to get). But the Centro is very simple, mechanically, unlike the LG Quantum. That slider is just one more thing that can go wrong in my opinion, and frequent use would only make it worse.
Which is a pity, since it seems that LG has a very nice speech-to-text app for their phone. Won't work with Word or Excel from what I can tell, so its not too big a loss for me.
Because the Samsung Focus can have its storage expanded, I'll be picking up a 16g micro-SDHC card to go along with it. That'll bump the storage up to a comfortable size. More than enough space for my files, music, etc. What's quite nice is I just found an app that might be ideal for audio-books. That'll be good, especially if/when Kobo gets around to making an eReader for WP7 (I buy my eBooks from Borders).