Need some fun once a while:

On dragging a non-connected node on a noodle, it will insert it.
Functionality tweaks are possible, but it already feels non-intrusive.

Rules:
- Insertion only when a single noodle is intersecting with node.
- Default connects first matching socket type. 
- If no socket match, it connects the first.

source/blender/editors/include/ED_node.h

@@ -39,6 +39,7 @@ struct Tex;
 struct bContext;
 struct bNode;
 struct ID;
+struct ScrArea;
 
 /* drawnode.c */
 void ED_init_node_butfuncs(void);
@@ -51,6 +52,8 @@ void ED_node_generic_update(struct Main *bmain, struct bNodeTree *ntree, struct
 void ED_node_shader_default(struct Material *ma);
 void ED_node_composit_default(struct Scene *sce);
 void ED_node_texture_default(struct Tex *tex);
+void ED_node_link_intersect_test(struct ScrArea *sa, int test);
+void ED_node_link_insert(struct ScrArea *sa);
 
 /* node ops.c */
 void ED_operatormacros_node(void);

source/blender/editors/space_node/drawnode.c

@@ -1869,10 +1869,17 @@ void node_draw_link(View2D *v2d, SpaceNode *snode, bNodeLink *link)
 		else {
 			/* check cyclic */
 			if(link->fromnode->level >= link->tonode->level && link->tonode->level!=0xFFF) {
-				if(link->fromnode->flag & SELECT)
-					th_col1= TH_EDGE_SELECT;
-				if(link->tonode->flag & SELECT)
-					th_col2= TH_EDGE_SELECT;
+				/* special indicated link, on drop-node */
+				if(link->flag & NODE_LINKFLAG_HILITE) {
+					th_col1= th_col2= TH_ACTIVE;
+				}
+				else {
+					/* regular link */
+					if(link->fromnode->flag & SELECT)
+						th_col1= TH_EDGE_SELECT;
+					if(link->tonode->flag & SELECT)
+						th_col2= TH_EDGE_SELECT;
+				}
 				do_shaded= 1;
 				do_triple= 1;
 			}				

source/blender/editors/space_node/node_edit.c

@@ -2492,6 +2492,127 @@ void NODE_OT_links_cut(wmOperatorType *ot)
 	RNA_def_int(ot->srna, "cursor", BC_KNIFECURSOR, 0, INT_MAX, "Cursor", "", 0, INT_MAX);
 }
 
+/* *********************  automatic node insert on dragging ******************* */
+
+static bNodeSocket *socket_best_match(ListBase *sockets, int type)
+{
+	bNodeSocket *sock;
+	
+	for(sock= sockets->first; sock; sock= sock->next)
+		if(type == sock->type)
+			return sock;
+	
+	return sockets->first;
+}
+
+/* prevent duplicate testing code below */
+static SpaceNode *ed_node_link_conditions(ScrArea *sa, bNode **select)
+{
+	SpaceNode *snode= sa?sa->spacedata.first:NULL;
+	bNode *node;
+	bNodeLink *link;
+	
+	/* no unlucky accidents */
+	if(sa==NULL || sa->spacetype!=SPACE_NODE) return NULL;
+	
+	*select= NULL;
+	
+	for(node= snode->edittree->nodes.first; node; node= node->next) {
+		if(node->flag & SELECT) {
+			if(*select)
+				break;
+			else
+				*select= node;
+		}
+	}
+	/* only one selected */
+	if(node || *select==NULL) return NULL;
+	
+	/* correct node */
+	if((*select)->inputs.first==NULL || (*select)->outputs.first==NULL) return NULL;
+	
+	/* test node for links */
+	for(link= snode->edittree->links.first; link; link=link->next) {
+		if(link->tonode == *select || link->fromnode == *select)
+			return NULL;
+	}
+	
+	return snode;
+}
+
+/* assumes link with NODE_LINKFLAG_HILITE set */
+void ED_node_link_insert(ScrArea *sa)
+{
+	bNode *node, *select;
+	SpaceNode *snode= ed_node_link_conditions(sa, &select);
+	bNodeLink *link;
+	bNodeSocket *sockto;
+	
+	if(snode==NULL) return;
+	
+	/* get the link */
+	for(link= snode->edittree->links.first; link; link=link->next)
+		if(link->flag & NODE_LINKFLAG_HILITE)
+			break;
+	
+	if(link) {
+		node= link->tonode;
+		sockto= link->tosock;
+		
+		link->tonode= select;
+		link->tosock= socket_best_match(&select->inputs, link->fromsock->type);
+		link->flag &= ~NODE_LINKFLAG_HILITE;
+		
+		nodeAddLink(snode->edittree, select, socket_best_match(&select->outputs, sockto->type), node, sockto);
+		ntreeSolveOrder(snode->edittree);	/* needed for pointers */
+		snode_tag_changed(snode, select);
+		ED_node_changed_update(snode->id, select);
+	}
+}
+
+
+/* test == 0, clear all intersect flags */
+void ED_node_link_intersect_test(ScrArea *sa, int test)
+{
+	bNode *select;
+	SpaceNode *snode= ed_node_link_conditions(sa, &select);
+	bNodeLink *link, *selink=NULL;
+	float mcoords[4][2];
+	
+	if(snode==NULL) return;
+	
+	/* clear flags */
+	for(link= snode->edittree->links.first; link; link=link->next)
+		link->flag &= ~NODE_LINKFLAG_HILITE;
+	
+	if(test==0) return;
+	
+	/* okay, there's 1 node, without links, now intersect */
+	mcoords[0][0]= select->totr.xmin;
+	mcoords[0][1]= select->totr.ymin;
+	mcoords[1][0]= select->totr.xmax;
+	mcoords[1][1]= select->totr.ymin;
+	mcoords[2][0]= select->totr.xmax;
+	mcoords[2][1]= select->totr.ymax;
+	mcoords[3][0]= select->totr.xmin;
+	mcoords[3][1]= select->totr.ymax;
+	
+	/* we only tag a single link for intersect now */
+	/* idea; use header dist when more? */
+	for(link= snode->edittree->links.first; link; link=link->next) {
+		
+		if(cut_links_intersect(link, mcoords, 4)) {
+			if(selink) 
+				break;
+			selink= link;
+		}
+	}
+		
+	if(link==NULL && selink)
+		selink->flag |= NODE_LINKFLAG_HILITE;
+}
+
+
 /* ******************************** */
 // XXX some code needing updating to operators...
 
@@ -2914,7 +3035,8 @@ void NODE_OT_delete(wmOperatorType *ot)
 
 /* note: in cmp_util.c is similar code, for node_compo_pass_on() */
 /* used for disabling node  (similar code in node_draw.c for disable line) */
-static void node_delete_reconnect(bNodeTree* tree, bNode* node) {
+static void node_delete_reconnect(bNodeTree* tree, bNode* node) 
+{
 	bNodeLink *link, *next;
 	bNodeSocket *valsocket= NULL, *colsocket= NULL, *vecsocket= NULL;
 	bNodeSocket *deliveringvalsocket= NULL, *deliveringcolsocket= NULL, *deliveringvecsocket= NULL;
@@ -3142,3 +3264,5 @@ void NODE_OT_add_file(wmOperatorType *ot)
 	RNA_def_string(ot->srna, "name", "Image", 24, "Name", "Datablock name to assign.");
 }
 
+
+

source/blender/editors/transform/transform_conversions.c

@@ -87,6 +87,7 @@
 #include "ED_object.h"
 #include "ED_markers.h"
 #include "ED_mesh.h"
+#include "ED_node.h"
 #include "ED_types.h"
 #include "ED_uvedit.h"
 #include "ED_curve.h" /* for ED_curve_editnurbs */
@@ -2182,6 +2183,12 @@ void flushTransNodes(TransInfo *t)
 		td->loc2d[0]= td->loc[0];
 		td->loc2d[1]= td->loc[1];
 	}
+	
+	/* handle intersection with noodles */
+	if(t->total==1) {
+		ED_node_link_intersect_test(t->sa, 1);
+	}
+	
 }
 
 /* *** SEQUENCE EDITOR *** */
@@ -4756,7 +4763,12 @@ void special_aftertrans_update(bContext *C, TransInfo *t)
 
 	}
 	else if (t->spacetype == SPACE_NODE) {
-		/* pass */
+		if(cancelled == 0)
+			ED_node_link_insert(t->sa);
+		
+		/* clear link line */
+		ED_node_link_intersect_test(t->sa, 0);
+		
 	}
 	else if (t->spacetype == SPACE_ACTION) {
 		SpaceAction *saction= (SpaceAction *)t->sa->spacedata.first;

source/blender/makesdna/DNA_node_types.h

@@ -179,6 +179,10 @@ typedef struct bNodeLink {
 	
 } bNodeLink;
 
+
+/* link->flag */
+#define NODE_LINKFLAG_HILITE	1
+
 /* the basis for a Node tree, all links and nodes reside internal here */
 /* only re-usable node trees are in the library though, materials and textures allocate own tree struct */
 typedef struct bNodeTree {