It's actually more than sliders. Separate square sliders do help reduce friction, but they aren't key.
If you look carefully, it looks like your rubber domes are individually molded.
There's two kinds of rubber domes:
1: Where the rubber domes are all molded together in one sheet:
IMG_0266 by
cfishy, on Flickr
(not my photo, all credits to cfishy)
2: Where rubber domes are individually molded and glued down to the membrane sheet. It's more apparent on the RT7D5JTW-model Dell Quietkeys, which have bright blue domes.
DSC_0501 by
Pylon757, on Flickr
(actually my photo)
From some old Compaq:
FLA_7043.jpg by
ripster55, on Flickr
(taken from Ripster's Flickr stream)
In general, the second type usually offers better tactile feel, though it also varies on the actual shape of the dome and isn't necessarily true always (my Dell SK-8115 feels decent for a rubber dome and it uses the first design). I have an old electrically broken Compaq mutimedia with an integrated keycap/plunger design but individual domes, and it feels great. However, the arrow keys happened to use different domes, and those particular keys on the board were very mushy.