How can the Torah write "Va'avad'tem Sham Elohim"?
Rashi, Ramban #1, Rashbam (both citing Targum Onkelos) and Targum Yonasan: It is referring to being subservient to the idol-worshippers - which is considered as if one is subservient to the gods themselves.
Ramban #2: With reference to what the Ramban wrote in Acharei-Mos, it hints that whoever lives in Chutz la'Aretz is under the jurisdiction of another god, 1 and it is therefore considered as if one is worshipping idols.
Why does the Torah not add the word "Elohim Acherim", as it usually does when referring to idolatry?
Ba'al ha'Turim: This hints at the fact that Yisrael in Chutz la'Aretz serve idols be'Taharah (even when they worship idols). 1
They may serve idols outwardly, but in their hearts, they believe in Hashem.
And why does the Torah see fit to list the four senses (hearing, sight, taste and smell)?
Ramban #1: It mentions hearing and sight, to point out that the gods that they worship are not deities at all - since they do not see their troubles or listen to their prayers, and adds that they are devoid of even the most physical senses of taste and smell, thereby rendering them inferior to those who manufacture them, unchanged from the stones that they were before they were manufactured.
Ramban #2: In contrast to Hashem, their gods are cannot see the troubles of their adherents or listen to their prayers, and that they are incapable of even emitting fire to consume the sacrifices that they place before them or to smell the fragrance of their offerings.
Seforno: The Torah is telling us here that he gods of the nations have no will of their own. 1
Seforno: Despite the fact that those who manufacture them make with the Keilim with which one expresses one's freewill (eyes, ears, a mouth and a nose), and worship them and pray to them in the hope that they will respond. In fact, the only creature that has freewill i man.