Of all of the High Elf special characters, Alarielle has the most special rules for her. That is probably fitting for somebody of her station, being one of the rulers of the High Elves. Alarielle was not in the last version of the High Elf book, but existed previously.
Alarielle is a Level 4 Wizard that is able to use spells from Life, Light, and High Magic. While she does not get to pick them like Teclis does, she can select how many of each type she wants. From the way that I read it, she can determine how many from each lore before the game. That makes it better than it would be if you had to note them on your army list. Also, keep in mind that she gets +1 to cast High Magic.
Alarielle provides any unit that she is in with magical attacks (ranged and melee), in addition to letting them ignore Fear & Terror. She also grants them a 5+ ward against non-magical attacks. So, if you pair these abilities with the Banner of the World Dragon, they would have a 5+ ward vs. non-magical or a 2+ ward vs. magical attacks. Not too bad.
She can heal one Wound a turn for a friendly model. She also has the ability to damage Daemonic units within 12″ of her. However, the presence of the Daemonic models affects her spellcasting ability, so you might not want her to get that close. If you do get her into combat, she can make a Heroic Killing Blow (with her 1 Attack) against an evil model. Once a game, she can cast a spell twice in the same phase, which might prove useful. This could be quite useful if you really need to get a spell off. The opponent could likely stop one casting, but not be able to stop another.
Including the Everqueen in your list is the only way to use a couple of the High Elf magic items. The Sisters of Avelorn can take the Horn of Isha, which gives the unit +1 To Hit for a turn. It doesn’t really seem worth the points to me. They have a fairly high BS and are re-rolling close combat attacks, so it shouldn’t increase the amount of hits by much. The other option is for the BSB to take the Banner of Avelorn. This one is more appealing since it boosts the casting value of Light & Life spells in the vicinity. However, your BSB will be fairly unprotected since he can’t take any magic items with it.
Alarielle is less of a magical powerhouse than Teclis, but has better army support abilities, which seems to fit her better. For all of the hobby gamers, the only true way to take the Everqueen would be in a massive army with large quantities of Sisters of Avelorn and some Handmaidens. After all, why would the Everqueen show up for a minor border skirmish?