I've never played 40k or WHFB, but I see nothing wrong with the Rings rules. They do seem a bit simple, but not overly so. The seige rules are fairly in depth. I haven't read the RotK rules yet, so I don't know what they've changed there (although Gil-Galad's new stats are much more appropriate now).
If GW's plan was to get me hooked on Rings and then get me to play their other games, I'm afraid they'll be rather dissappointed. I have next to no interest in the other games. They're neat, I'm sure, but why would I spend money on them when there's a perfectly good game based on my favourite book?