See Oldchapel.

Huh. I would let a lot of spells and abilities target me if it meant that I didn't lose 1 life per turn. There's some real potential for 'feel bad' here, that isn't quite present with Oldchapel.

(Also, this is one of those cards that's bound to trip up new players that activate effects during their turn. As an aside, if all the card is doing is gaining life, I like to do that on my turn since there's no real value to activating it on the opponent's turn. But these are minor considerations, admittedly.)

Originally, I was also going to comment on Wizard's in-house rule that lands always produce mana. But this already does... it's a Plains... which is hard to notice, so it probably requires reminder text, like on Leechridden Swamp (if you ever moved this out of temporary storage, that is.)

I'd also argue that this is overpowered, but I think Leechridden Swamp and Mistveil Plains kind of do that job for me. Put another way: I think I would accept an additional loss of four life as part of the cost of playing this card. Maybe that's too much... it depends on the speed of the format this card is played in. And I'm not saying you should add that line... you get the idea.

I like the design quite a bit.

I think the lifegain is a bit much, if you can get mostly-free lifegain, it's easy for that to dent the opponent's ability to attack you.

Conversely, I see jmg's point about giving up the lifegain to get the effect, but it feels like, even beginners can look at an effect and judge if they'd rather have the lifegain (whether or not they're right). If you just counter any spell targetted at you, that's probably a mistake, but you can see how scary it looks.