1) Guardian hosts are most useful if they have a couple ranged shots for laying BMs (blast markers). It's standard around here to trade out for some heavy weapons if you want a "stripped down" host for cheap.
Another common strategy is to add Wraithguard to give them a big boost to durability. Having the first couple shots hit those high-armor units will keep the formation a threat much longer. Of course, you pay for it, but most people think they are worth it.
Another option, though I don't see it much personally, is Guardians in Wave Serpents. The Wave Serpents provide ranged fire if you need to place BMs and the formation overall is strong in FF assaults.
Aspect warrior formations are usually tooled for CC or FF (close combat or firefight) to maximize the effectiveness of their specialization, but there are a few people who like them general purpose. Common compositions are things like....
6 Dire Avengers + 2 Dark Reapers w/ Exarchs - lots of FF. Although it's fewer FF attacks than all DAs, the extra attacks on the exarchs at the high FF value of the Reapers makes up for most of it. It has the additional advantage of a few ranged fire shots if you need to place BMs.
4 DA + 4 Warp Spiders w/ 2 Exarchs - again, an FF formation. Fewer attacks, but the Spiders and the attached Exarchs get First Strike, and they have better armor.
8 Scorpions + 2 Exarchs - CC. CC units are always optimal with some sort of transport or deep strike option. In this case that means Wave Serpents, Vampire Raider or a Storm Serpent portal.
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2) I don't think any of the units are hard to understand, but the overall flow of Eldar play has some nuances that might not be apparent. I would suggest doing a forum search for "rolling assault" and find a description how to set one up. It's effectively the Epic Eldar signature move, using a combination of "Hit and Run" and "Farsight."
Also related to assaults, staying in transports is less of a death trap for Eldar than other forces, as they can withdraw fully after an assault. A true hit-and-run style is workable with mech infantry and other fast assault units (bikes and the fast aspects).
There's a thread in the
Strategy and Tactics forum. It's more of an ongoing discussion than a primer, but it may have some good info for you.
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3) There are only a few things people give a pass to.
In the Biel Tan list (BT), the Void Spinner is generally considered better than the Night Spinners for arty support, even at the higher price, so people skip the Night Spinners.
Howling Banshees have never quite held their own with the other Aspect Warriors.
There is a faction that believes the Phoenix Bombers are too expensive and points to the fact that they are seldom seen in competitive lists. The other side points out that the Bombers are 400 points, which is a big chunk of the "restricted" points at the normal tournament levels (~3000 points), so they aren't seen because of that rather than because they are inherently weak.
Nearly everything else makes at least semi-regular appearances.
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Edit: I'd also recommend reading Jaldon's basic
"how to build an army" article for general game strategy.