If the fighter had line of effect to the target, then the bonfire can't have been all that big. The target would most likely have been able to see him through it.
In which case there's no reason why the target might not have decided on a terminally heroic "leap through the bonfire".
In general I agree, but these circumstances were rather specific. All I'm saying is that, used sparingly, saying no is a viable option, especially if you aren't a jerk about it. Honestly, if a particular use of a power would
reduce the fun around the table, I think it's perfectly okay to disallow it.
In the case I stated (yes, it wasn't technically a
giant bonfire, merely a sizable one) we had a fighter who was mostly surrounded but had put his back to a 5' bonfire. The bonfire was very dangerous (something like 3d6+5 fire damage when entered and every round one remained), difficult terrain, and granted cover (but not total cover). The target in question was a bloodied troll leader who had just managed to disengage himself from the party fighter after taking quite a beating and had retreated to the other side of the fire. Due to the setup of the field, the only legal space that he could stand in if he moved adjacent to the fighter had one of his squares in the fire.
The fighter said he was using Come and Get It, and looking at the scenario I just couldn't see how he could taunt or trick a
troll of all things to stand in the fire, despite that I could have probably used it as the basis for a halfway decent Warren Zevon pun (trolls are none too bright but fire is one of the only things they actually fear, which was why the party had made the bonfires in the first place). I asked him if he could think of some rationale for it and his response (to paraphrase) was, "Yeah, I hadn't really thought of it, but that would be kind of stupid; can I do something else instead?" to which I responded, "Of course".
CaGI isn't mind control or even an ultra-brilliant warlord stratagem; it's a trick/taunt. I wouldn't consider a pit a legal adjacent position, and this seemed pretty close to the same thing.
Well, using hazards and the terrain strategically is a part of smart playing the rules, if you try to rationalize the functioning of some powers, especialy those that make forced movement which the Warlord class has in abundance, and prevent them to work you will be crippling the players ability to make strategic decisions.
Btw, there is a rule for that exact situation. The creature would get a saving throw and drop prone as the bonfire qualifys precisely as being Hazardrous Terrain, its the same situation as if it were the edge of a cliff.
Actually, it's a bit of a grey area I think. CaGI isn't technically forced movement as defined by the game (push, pull, or slide) but rather a shift that must be performed in a particular manner.
The fighter has used it at least a dozen times, maybe even two dozen, over the course of many games to great effect. It's an amazing lock down power. This one time I ruled that one target (the troll who'd have to stand in a blazing inferno to get to him) would be immune to effect. The other targets were still viable, and I informed him of this before the action was resolved and allowed him to use a different power instead.
IMO, I'm not exactly some killer DM out to "cripple my players' ability to make strategic decisions". (I am an evil DM, but that's got everything to do with my tendency for making puns while behind the screen, rather than anything like trying to screw over my players.) I'm just doing the DM's job of making judgment calls and trying to maximize fun for everyone at the table. It's possible that I made a mistake and my players would have found a troll jumping into a bonfire to be totally awesome, but I think I know my players well enough to say that it's more likely that we'd all have noticed it was a bit stupid and been annoyed. In over a year of DMing, this was the first time (at least in my recollection) that I've said no in this respect.
While a good DM should strive to say yes (but), I don't think there's anything wrong with saying no once in a while. If neither you nor the players can think of a reasonable rationale for something, it might be time to consider saying no.