There are a few points to keep in mind with Protosuits. They've been (mostly) covered already, but have a compiled list:

1. Many players preferentially target Protosuits. Unless there are several Protos on your team, you WILL be a priority target just by wearing it. People want to see your death. This is partly because they know Proto gear is expensive and they're costing you a lot of money, and partly because we're all sadistic little kittens. The fact that your death improves the team's situation more than a Militia guy's death is just an added bonus. Another point is that the "pot" earned by players for participation in the battle is partly based on the ISK value of gear called in. Killing a Protosuit frequently means another one will be called in straight away, which in turn means that profit margins go up for everyone involved in the battle (except the guy losing his Protosuit, obviously).

2. Protosuits aren't invincible. The players who make Protosuits look invulnerable can make Advanced gear look almost indestructible as well.

3. Many Proto players unconsciously try and take on worse odds than their suit can actually handle, throwing themselves into suicide attacks because "it's ok, my protosuit can handle it". I've seen Prototype Assaults rush at me with my Militia Shotgun because even though it's a Shotgun, and even though they know I'll be waiting RIGHT THERE as they turn the corner, they somehow think it's going to be fine to run right after me. Usually, I kill them for that mistake, and even if they do take me down, they lose all their shields and most of their armour, making themselves an easy target for my squadmates.

Typically, when I see a proto suit wandering around alone, I let the entire team know. Some avoid the area entirely and attack a different objective but most will wander over and feed the proto until he gets suicidally overconfident... or sniped. Sniped works too.

I've lured you into my Shotgun more than once, but once you know what I'm running, it stops working.

The Shotgun was an obvious example I see a lot, not something ALL Proto players do. Good players are aware of their vulnerabilities, and know how to compensate for them.

I actively hunt proto suits. My forge gun doesnt hold that many shots so got to make them count ... and 1 shot killing proto heavies is hilarious

I think my favourite moment like yours is...

"Was that guy carrying a Shotgun? Not sure... lets find out!"
*BOOM*
"Yeah, that was a Shotgun."