There are three things that stand out to me about this guy:
It’s highly complex for a common. A casual or inexperienced player is likely to glance at Leyline Phantom and think, “Ooh, a 5/5 for 5! Oh, too bad it returns itself; that sucks,” whereas a more experienced player is likely to glance at it, and immediately have an aneurysm from considering too many possibilities at the same time. And "head-plosion" is the correct response: the Phantom is one of the most synergistic, complicated, limited pieces I’ve played with.
It’s a powerful card for any Simic deck, since it can be cast repeatedly to trigger evolve, as well as a team player in an Esper Extort deck, since it’s a strong early game defender and provides a repeatable spell to trigger Extort. It also has synergy with Sage’s Row Denizen to set up Dimir mill effects, and directly combos with Consuming Aberration. And even in a deck with none of those interactions, it’s still an annoyingly efficient attacker and blocker that potentially has pseudo-vigilance, which is obnoxious for opponents to play around. Considering its numerous interactions and its utility in Limited, I’m a little surprised Leyline Phantom wasn’t an uncommon; I’m guessing the reason it stayed at common is that drafters are likely to fight over it, and it’d be awkward to have multiple players in a pod drafting different strategies still end up needing the same card.
The template is unique. Combat damage triggers typically specify either to creatures or to players (or opponent, in the case of Hydra Omnivore). Besides the Phantom, the only other two creatures that trigger from any combat damage are Drinker of Sorrow and Sunhome Enforcer, and the enforcer barely counts since his ability is just a poor man’s lifelink. What makes Leyline Phantom unique is that its trigger affects itself. Again, combat damage triggers typically either affect the creature being dealt damage to (if it’s a creature trigger, like Lowland Basilisk), or one of the players (if it’s a player damage trigger, like Bloodcrazed Neonate). The unique template sets up the coolest part of the card, which is the reminder text. Getting to use the word “survived combat” is a direct result of the space opened up by using “dies” instead of the old put-into-the-graveyard template, and it gives Leyline Phantom the same kind of functional-form aesthetic as Obsidian Fireheart, which I thought rocked. Not to mention the best part...
The reminder text opens up new space. The “survived” phrasing is a neat experiment, and maybe if it works and players dig it, it’ll show up in the 2014 and 2015 blocks. It’d certainly be helpful to have another creature ability for blue, especially since blue creatures still tend to encourage players to sit back and turtle when they should be attacking. A combat survival mechanic also seems like it’d work in red, since it sounds thematically appropriate for a color that likes fighting, and like blue, red tends to come up short on creature mechanics. The first example that popped into my head was that it’d make a great trigger and reminder for a flip card, like if Bushi Tenderfoot / Kenzo the Hardhearted was re-concepted as a double-faced card. I like the idea of a white citizen turning into a red warrior after his or her first taste of battle. The other thought that occurred to me was that Olaf’s enrage mechanic could be easily repurposed to work with survives:
Enrage (When this creature deals combat damage, put a +1/+1 counter on it. ~ gains a counter only if it survives combat.)
Using this version of Enrage makes for some fun puzzles in deciding races and when to attack or block, and opens up the chance for some sweet combat tricks involving first strike and damage prevention.
More generally, the new template gives aspiring designers a new tool to tinker with. The actual guild mechanics from RTR block operate in a fairly narrow space, which is fine for playing with, but less good for getting the creative juices flowing. So I'm glad Leyline Phantom opened up some new areas to build in, and I'm curious to see what else you all can do with it!