Pre-flop raising is important in Omaha H/L, but it depends on position and likely responses of other players. Some tables pre-flop will often be capped, but not most of the ones I've played on. A passive table can be very good, because then you can either raise to thin the field, or get in cheap to see a more speculative hand (like a weak A2-type hand).
There is a common mistake of thinking that because the pre-flop hands are closer in value, and there are so many more combinations possible, that you should play more hands... the opposite is true - because while in Holdem you're looking to improve, and 2 pair is often a winning hand, in Omaha 2 pair is generally junk, even top 2 pair, because of the number of combinations each player has. As a result you want to look for hands which have more likelihood of making the best possible hand.
Flushes are still of use, but they're not particularly bettable unless they're Ace-high, when playing High-Low, playing the 2nd best low hand is a suckers game (unless you have the nuts for the high hand as well or suitable draws) and so on... so you pick cards that are more likely to give you either the nuts or nothing. A2XX is therefore much better than 23XX in High-Low, because you can make a nut high hand with the Ace, AND it's much more likely to make the nut low rather than 2nd best.
And as has already been mentioned, never ever forget that it's just 2 from your hand, otherwise you'll bet a straight or flush that you don't have, and this a mistake that I think all regular Omaha players have made at some point or other, and can be expensive (particularly in Pot Limit).
